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1
2
3
1
2
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4
5
6
PUTE XI.
TIIK NOK
CIKCUMPOLA
TIIK NOKTH
OIKCUMPOLAll STARS
{Fr^m Diek't Anlroaomi/,
■J •.;.■ i iVf-:
iA
i
m
ITie position of (lie Planetary Axis relative to that of the Primary.
(a.) Parallel, (h.) Transverse, {d.') Perpetidicular .
(a ) The general arrangement of the axia in the planets belonging to the solar Bystem.
M") The exceptional arrangement in the case of the planet Uranos.
i
(3.) CEJ^TRIFUGAL FORCE 4- GRAVITATIOjY'
THIO O.RY
or
THE STELLAR UjYIVERSE.
AM)
The Mixed Doctrine of
PARALLAX AND ABERRATION.
BY
JOHN HARRIS.
■--•.:' X
PRINTED BY THE LOVELL PRINTING AND PUBLISHING 00
St. Nicholas Street;
May. 1875.
i73'f
^
0-
r
t.z
INDEX
'Aoa
Chap. I. — The Solau System and the general
ARRANGEMENT OP THE SiDEREAL UNIVERSE.
(1.) The present state of Astronomical Science 9
Horizontal System of Astronomy 11
(2.) Stellar Systems having their axes of revo-
lution perpendicular to that of the Solar
System 13
(3). The planets Uranus and Neptune, and the
question of a neighbouring Stellar system . 15
(4.) Masses of aggregated matter and their rela-
tion to the laws of the material universe. . 23
(5.) The relative distances of the visible stars.. 27
Chap. II. — The Present mixed doctrine op
Parallax and Aberration.
(G.) Theory of Parallax 30
(7.) Theory of Aberration 48
(8.) A direct method of obtaining parallax of
the distant stara 68
8 INDEX.
Chap. UI. — The AuEiuarioN-ot-LiGnT Tue-
OKY KXAMINEl). *
(0 ) The conditions and requisitions of the
theory 70
(10.) Practienl Jipplication of the theory 72
(11.) The nature of Light as assumed by the
theory (Immaterial Matter) 7-5
(12.) Aberration a dynamical theory 77
Note a. The dynamical theory not in har-
mony with the laws of motion
(13.) Distrust in the gift of sight required by
the Aberration theory 80
Note b. A further dynamical test of the
theory 82
Note c. The continuous radiation of light
and heat into space. . . A. e., radiation to
waste. . . .required by the theory 83
(14.) Direct heliocentric methods of obtaining
parallax of the distant stars 85
I
INDEX OF PLATEH.
Ploto 11. Illustrating tho Circumpolar Stara... [ pjg"Q"\'
Page
Pinto 12. Illustrating... Tho Equinoctial and Ecliptic. 88
" 13. ^* Early Systems of Astronomy. 10
" 14.) „ • ( Tho Theory of tho Stellar Uni-
" 16. J ( verso 22
t. in „ J The Theory of Cometary
'^^' I Orbits 24
i Tho position of tho Axis of")
planetary bodies in ro- ^Jf"^"!;
lacioii to their primary.... )
Plato I « f Uranus considered m a Solar
Pig. 6. 1 I Planet 20
Plato 1 t< J Practical application of tho
Pig. 11. j,^ \ Aberration Theory 74
Plato I .. f Method of obtaining helio- V
Fio- 12 f "i centric parallax of the dis-
*= ) (tant stars 86
^pfff 13 } " Aberration and Parallax 78
Plato 6. Fig. 10 belong f The Solar System and the
ing to Part Second. ( Polo Star 12
i>i„+^ 1 v A V ( Tho deviations of Uranus
Plato 1. Fig. 4. From \ ^^^^ ^^^ supposed Solar
Herschel 8 Astronomy, j Orbit 92
on
h.i'i
•If
.£1 -ai'l
;j .• I
•'M''*
M!'i 0)
CHAPTER I.
The Solar System and the General Arrangemmis qf the
Sidereal Universe.
INTKODUCTOKY OBSERVATIONS.
(1) The present state of Astronomical Science.
If we go back to that epoch in human education
which may be termed the childliood or early age of
astronomical science and make comparison, we find the
general apprehension of the relation existing between
the particular stellar system to whicli we as terrestrial
beings belong, and the sidereal universe, to be, at the
present time, in some important respects, much more
distinct and based in a more considerable degree upon
a natural foundation of I'eality and certainty, but, in other
respects, also very important, we find the ground occu-
pied by doctrines not wholly consistent with those gener-
alizations of experience and fact to which they pertain,
and by theories some of which are as artificial and unreal
in character as any, perhaps, of those taught at the
earlier period.
it is ifue the fact has been now long known that the
eun and not the earth occupies the centre of the solar
«ystem ; much precise and accurate knowledge lias been
B
INTRODUCTORT OBSERVATIONS.
!
obtained as to the dynamical relations of the various-
members thereof to each other ; much has been done in
ascertaining and permanently recording the relative posi-
tions of the more distant celestial bodies ; much progress
has been made in the formal and rigorous (mathematical)
application of the sciences of force and motion (Mechan-
ics), of magnitude and form (Geometry), and of number
and quantity (Algebra), to the observed phenomena of
astronomy ; great improvements have been effected in
the instruments which enable or aid the astronomer to
correctly observe those phenomena, and experience has
made evident the importance of systematic observations
by trained and practised observers to ensure correctness
and accuracy in the general record of the observed facts
belonging distinctively to astronomical science.
Notwithstanding, however, the great advance whicb
has undoubtedly been made in these particulars, the pre-
sent state of the astronomical department of general
science may be considered an intermediate station be-
tween the old (artificial) system and a new (natural)
system rather than as constituting in itself a complete,
coherent, and intelligible system. Such as it is, it may,
for reasons which we shall immediately proceed to
explain, be distinguished by the appellation of. . ' the
horizontal system.' It may be described as consisting in
part of an imperfect natural system — i. e., of a sound
system based on reality and fact, and in part of the old
artificial system which, although nominally and formally
discarded, still retains its hold on a not inconsiderable
portion of that domain of which it formerly held exclu-
sive possession.
I'Laii 13
./
. r
V yfvfliuni Mobile
Sniurn
«• juptter
^V(lr.s
' . Sun.
\ \
\ \
N.
^:
- / ,.
-^
,'/../' '-v
'i;\\«'
tvr^''-^*^:.^;r.^^^«'i .9,,,.^^
.lupiter
MCl-s
*
*
\ ■Ad'
\
■^
Frcrrt
T/if Sysiern o/' Tyrho Brnhp .11). 1580.
f.ncyc. Itritanniia.
BW
"-^1
The Horimiidl System of Askommt}.
Hipparclms, of Rhodes (140 years b. c), is, perliaps,
entitled to rank as the last, as well as one of the greatest
of the astronomers belonging to the older civilization ;
for T'tolemy (a. d. 130) was more a recorder of the
progress already made and a connecting link between
ancient and modern astronomy than himself an original
observer or discoverer.
Now the ancient system, of which Hipparchus was the
most advanced exponent, represented the various celestial
bodies as revolving in concentric circles, within which the
earth, already posited obliquely with the axis Inclined to
that of the sun, occupied the actual centre ; See Plate 13.
Amongst the later of the ancient mathematicians Plato,
in particular,is supposed to have suggested the theoretical
representation of the planetary orbits by circles in the
same piano, and, the resalt appearing to harmonize well
with the observed phenom>^na, the inference appears to
have been at once adopted that the dynamical orbit of
each planet or moving star, must be an undeviating hori-
zontal plane, and this inference seems to have included
the assumption that one uniform horizontal plane was
common to all the planets of the solar (or terrestrial)
system. *
* Whether la aaclent astronomy any inference was arrived at as to the
obliquity ot the orbits of other planets, or as to the perpendicularity or incli-
nation of their respective axes, does not appear ; but in modern astronomy,
even to the present time, the doctrine of the inclined axis leo ves it open to
the astronomical student to suppose all the planetary orbits to bo confined
to the one uniform horizontal plane of the ecliptic, (as shown in iig. 23 R,
belonging to the preceding part of this series), the axis of the planet hav
ing, in each case, its own especial and distinctive inclination. It is true
the practical astronomer, at the present time, would, if he CDrefuUy con-
12
HORIZONTAL SYSTEM OP ASTRONOMY.
So proiuineiit a pheiiomouon as the undulating path
of the sun in the heavens during its annual revolution
could scarcely fail, however, to attract the notice and
attention of astronomers at an earlier age than the com-
paratively advanced epoch of PVito and Ilipparchus ; we
find it recorded, accordingly, that Tliales, of Miletus ((i'lO
years b. c.)j the lounder of the Ionian school, either
discovered for himself or obtained information * that
the equatorial plane of the earth is cut obliquely by the
ecliptic ; but tiie assumption that the earth's axis of rota-
tion was inclined to the axis of the sun appears to have
been proposed and accepted as a satisfactory explanation
at a very early date, and the prejudice that an oblique
position of the earth combined witli a revolution in a
horizontal plane was equivalent to an oblique orbit f
having established itself as a postulate or a (supposed)
demonstrated theorem of astronomical science as then
tauglit, became inlierited and accepted as a part of modern
astronomy, without suspicion.
aidercd the case without prejudice, at once know that such cannot be the
actual arrangement, for, if it were true, eclipse, occultation, or transit would
necessarily take place each time of conjunction between a planet revolving
in ihc outer circle and an inferior planet; but, on the other hand, there is
the obvious obliquity of the sun's apparent path as seen from the earth, and
if that can be accounted for by supposing an inclined position of the earth
revolving in a horizontal orbit, it is evident that the same explanation
would apply to the case of each other planet supposed to revolve in a hori-
zontal orbit.
* It is recorded that Thales, as Plato and others of the Greek astro-
nomers subsequently did, travelled into Egypt expressly to obtain inform-
ation from the Egyptian priests on scientific subjects.
t Observe that if the case were confined to the earth and sun only, and
the position of the sun's axis be considered indeterminate or optional by
the theorist, the obliquely posited earth and horizontal orbit would actually
be equivalent to the oblique terilestrial orbit. . , \
r
iM
■'■»;
itiiig path
revolution
lotice and
I the com-
rchu8 ; we
hjtus (040
ol, either
ion * that
cly by the
ds of rota-
irs to have
xplanation
in obhque
ution in a
lie orbit f
supposed)
JO as then
of modern
cannot be the
transit would
met revolving
hand, there is
the earth, and
n of the earth
le explanation
olve in a hori-
■M ■
Greek astro-
>btain iuform-
sun only, and
)r optional by
nrould actually
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(2.) Stellar systems having their axes of revolution
perpendicular to that of the solar system.
The effect of a stellar system having its orbitnl plane
perpendicular, or inclined at some considerable angle
to the orbital plane of the solar system, does not seem:
to have been made the subject of especial study by astron-
omers. In our Fig. 10 (PI. 6^, illustrating the pole star
and the solar system, the distance of the pole star from
the sun as represented is less than twice the distance which
the planet Uranus (if represented) would be from the
sun. Therefore, if we suppose, for the purpose of illustra-
tion, the pole star to be the centre of gravitation of a
system, having its central axis directly perpendicular
to that of our solar system, and of which the orbit
of one of the planets was about equal in diameter
to the orbit of the planet Neptune, it is evident that tiie
planet so circumstanced would approach more or less
closely to our sun. In looking at such a rciiresentation
as that shown at Fig. 10, and on a merely superficial con-
sideration of the case, it would seem that such a vertical
motion of a body at right angles to the path in which the
earth is moving, if seen from the earth, could not be mis-
taken for or confounded with a motion in the same or
nearly in the same plane as that of the earth's revolution,
A closer and more attentive consideration of the actual
conditions, however, will show that it might not be vcrv
difficult to fall into such an error. The planet would
have descended, so to speak, more or less nearly to the
horizontal pLne before the light from our sun rendered
it visible to an observer on the earth, it would th m
appear to approach the sun from or in an almost horizon-
tal plane ; and, if instead of the directly vertical, we sub-
14
STELLAR SYSTEMS.
stitute the supposition of a plane apparently deviating very
considerably from the vertical, we shall then have a case
wherein the compounded motions would be very likely
to perplex and mislead an observer whose point of view
was upon the earth ; and such would be almost certainly
the result, if tiie observer viewed the moving body with
a prejudice or foregone conclusion that the body was
revolving around our own sun in a plane either horizon-
tal or not deviating very mach from a horizontal plane.
Let us consider some of the conditions under which a
planet, belonging to a system having its axis perpendicular
to a vertical plane passing through its centre and through
the centre of the sun, would present itself to a terrestrial
observer. In the first place, since the motion of the stran-
ger planet would be at right angles to that of the earth, the
actual orbital motion of the earth would in appearance be
transferred to the planet, and would become an addition
to the actual motion of the planet, thereby converting
the vertical into an apparently oblique motion. If the
stranger planet was of considerable size and approached
sufficiently near to any of the planetary members of the
solar system, it would perturb or cause a deviation in
their orbital motion. If attended by satellites or moons,
these would have an apparently oblique motion of revo-
lution around their central planet in tlie opposite direc-
tion to the orbital motion of tiie earth. It is evident that
— so long as the two systems retained the same relative
positions, and the distance between the sun and the star
remained the aa.h:?t — the stranger planet would periodi-
cally return in its orbit of revolution around its own
centre of gravitation to the same relative place ; and
hence, particularly if the distance was very great, and
,!..
iii
i
STELLAR 8T8TEMS.
IB
iiting very
ave a case
ery likely
t of view
certainly
)ody with
body was
r horizon-
tal plane.
' which a
pendicular
d through
terrestrial
the stran-
earth, the
jarance be
n addition
ion verting
11. If the
pproached
lers of the
viation in
or moons,
11 of revo-
•site direc-
ident that
le relative
d the star
d periodi-
i its own
lace ; and
great, and
observations required the medium of a powerful tele-
scope, the stranger planet might be very easily mistaken
for an additional member of the solar system. Enter-
taining the not improbable supposition that other stellar
systems may be so arranged as to have their planes of
revolution vertical to, or differing considerably from,
that of the solar system ; and that some of the members
of one or more of these stellar systems may be, or
become by the aid of very powerful telescopes, visible
from the earth, let us consider the case of the two most
distant planets which are now supposed to belong to the
solar system. In doing so it will be most satisfactory to
take the least distant of the two, as being the best
observed and of which the apparent motion, for some
considerable time past, has been recorded.
(3.) The planets Uranus and Neptune, and the question
of a neighbouring Stellar system.
The Planet Uranus. — Subsequently to the discovery of
this planet by Sir Wm. Herschel in 17SI, it was found that
observations of it had been recorded by preceding astron-
omers, and that its progress could be thereby traced back,
witii some degree of certainty, to the earliest period of
such observations. This having been done, the result
showed that the actual orbital path, through which the
planet had (appeared to have) moved, differed greatly
from the theoretical path which, considered as a member
of the solar system, it should have followed. By attri-
buting possible error to the earlier observations, and by
theoretical suppositions of more or less ingenuity, the
discrepancies were greatly reduced, and the motion of
the planet was thus made to seemingly harmonize with
that of the solar system until the year 1805, from which
16
THE PLANET URANUS.
time till the year 1822 the departure of the planet from
its supposed orbit became so marked as to suggest a
search for some sufficient cause of such apparently un-
accountable disturbance. The result of this search was
the discovery of the planet Neptune. There can be no
question as to the result being highly creditable to the
perseverance and industry of those concerned in the in-
vestigation ; but as to the precise nature of the result
in a scientific sense, there is a great diversity of opinion
— some considering it a great astronomical and mathe-
matical achievement, because the planet was found in
consequence of and very near to the actual place indi-
cated by the calculation ; but by others, viewed as being
to a certain extent a merely fortuitous coincidence (a
lucky chance), because wlien discovered and actually
observed, the elements of the real planet were found
to differ greatly (enormously) from those which had
been assigned to it as the result of the hypothetical
computation. The accompanjring diagram, Fig. 4
(PL 1), copied from HerscheVs Outlines of Astronomy,
shows the discrepancy between the theoretical and ob-
served path of the planet Uranus from the year 1690 to
about 1845. (See the Appendix.)
The actual discovery of the planet Neptune having
confirmed and apparently verified the conclusion that
the motion of Uranus, in its departure from its sup-
posed orbit, was to some extent effected by such source
of local gravitating influence («?> was perturbed by Nep-
tune's attraction), seems to have occasioned a forgetful-
ness on the part of astronomers as to the previous lesser
but yet very considerable discrepancy which had been
partially reconciled by the ingenious but somewhat
''^ii
THE PLANET URANUS.
IT
net from
iuggest a
jntly un-
irch was
an be no
le to the
n the in-
le result
opinion
d mathe-
found in
ace indi-
as being
idence (a
actually
re found
hich had
jothetical
Fig. 4
stronomy^
I and ob-
r 1690 to
le having
si on that
its sup-
ch source
i by Nep-
forgetful-
ous lesser
bad been
omewhat
'4!
violent suppositions already mentioned. If, however,
this partially reconciled discrepancy were the only
remaining cause of doubt, it would not have been sur-
prising that astronomers should, under the circumstances,
consider the case, for the time, as satisfactorily explain-
ed, and rest satisfied accordingly ; but there is another
known (observed) circumstance belonging to this planet
Uranus, so remarkable from its exceptional character as
to forcibly suggest itself as an independent reason for
the exercise of great caution in admitting the newly
discovered planet (Uranus) to be a member of the solar
system. The circumstance is that "the orbits of these
satellites (the satellites of Uranus) offer remarkable and
indeed quite unexpected and unexampled peculiarities.
Contrary to the unbroken analogy of the whole planetary
system — whether of primaries or secondaries — the planes
of their orbits are nearly perpendicular to the ecliptic,
being inclined no less than 78° 38' to that plane, and in
these ^orbits their motions are retrograde; that is to say,
their positions, when projected on the ecliptic, instead of
advancing from west to east round the centre of thf-ir
primary, as is the case with every other planet and
satellite, move in the opposite direction. Their orbits
are nearly or quite circular,and they do not appear to have
any sensible, or, at least, any rapid motion of nodes, or
to have undergone any material clmnge of inclination,
in the course, at least, of half a revolution of their
primary round the sun. When the earth is in the plane
of their orbits, or nearly so, their apparent paths are
straight lines or very elongated ellipses, in which case
they become invisible, their feeble light being effaced by
the superior light of the planet, long before they come u]>
18
THE PLANET URANUS.
to its disc. So that the observations of any eclipses or
occultation they may undergo is quite out of the question
with our present telescopes." {HerscheVs Outlines of Astro-
nomy'). The observed facts hei'ein recorded, if the suppo-
sition that the planet Uranus belongs to the solar system is
retained,appears even more remarkable and extraordinary,
when the circumstances of the case are submitted to a
particular examination ; because the fact of the satellites
or moons revolving around the planet in a plane perpendi-
cular to the plane of the solar system, almost necessitates
the inference that the planet itself must rotate on an axis
parallel to the plane of the solar system, and thus, on
the supposition that Uranus is a solar planet, we have a
departure from what may be called the plan of the sys-
tem, considerably greater than at first sight appears. Is
mechanical science sufficiently advanced as yet to decide,
by reference to experimental demonstration (i. e., by the
record of reliable and unobjectionable experiment),
whether such arranorement would be mechanical)'
admissible ; that is to say, whether, according to the
laws governing mechanical forces, such arrangement
would have the necessary quality of stability ?
The arrangement would admit of three forms ; namely,
the horizontal axis, on which the revolving planet
rotates, might have (1) a position at right angles to a
Tertical plane joining the planet and the (centre of gra-
vitation) central body of the system ; or, (2) it might be
situated obliquely to such a plane ; or, (3) one extremity
of the axis might point directly towards the central body.
Such three forms of the arrangement are indicated in
Fig. 6. (Note. The axis of the central body of the
system, supposing it to rotate, is understood to be per-
THE PLANET URANUS.
19
pendicular to the nodal plane of the planet's revolution.)
We do not think the case can be authoritatively decided
by reference to experiment, but we do not hesitate to
express a strong opinion that neither of the forms of the
arrangement vi^ould be permanently stable ; the axis of
rotation of the planet, in such a case, v^^ould more or
less gradually assume a vertical position ; that is to say, it
would become perpendicular to the sun's equatorial plane.
If, however, we admit the assumption that the planet
Uranus belongs to the solar system, we then have the
form of the arrangement defined by the observed fact as
recorded by Herschel* to be similar to that of (1) in the
*This is manifestly included in the statement of the (observation)
already quoted. Owing to the great distance of Uranus from the sun, the
line of vision from the earth, in any relative positions the moons can
occupy, will be very nearly the same as if the planet was viewed from the
sun : consequently the recorded observation necessitates the inference
of the planet being posited as shown at (1) in the figure.
20
URANU8 AND NEPTUNE.
: ti
I
figure — namely, with the axis of the planet at right
angles to a vertical plane joining the planet and the sun.
The difficulty as to admitting the assumption is increased
by taking into consideration the moons or satellites of the
planet, as shown at (d) in Fig. 6 (PI. 2) ; the angular orbi-
tal velocity of each moon would be greater than that of
the planet when inside the orbital circle, and less when
outside the planet's path ; the difference would be very
small, but there would necessarily be a continual and
active tendency of the moons towards the horizontal plane
of revolution. Now if we take the assumption that the,
planet Uranus belongs to a stellar system having its plane
perpendicular to that of the solar system, the same very
important and interesting fact observed and recorded by
Ilerschel (quoted at page 17), v/ill also serve to indicate,
if not to define, the relative position of the central body of
the neighbouring system, viz. thes^^r, to that of our sun ;
because it is at once evident that the plane of the planet's
Mhit must coincide (or nearly so) with a vertical plane
joining the central star and the sun ; for if it does not,
let it be supposed that the plane of the planet's revolution
is at right angles to the vertical plane joining the star
and the sun; then, observation would show the planet's
moons revolving as at (6) Fig. 6 (PI. 2), whereas the
fact is recorded to be as at (a) in the same figure ;* and
similarly the supposition of more than a slight deviation
(j. e., a moderate degree of obliquity) from the plane join-
• The meaning inteuded, as to the relative positions, may be defined
by stating that the vertical plane j.)ining the sun and star coincides with
the equatorial plane of the srar.
!-ti»'.
I'hilr Fill (I
Vnnin^ coiia'nh'rrd oh n Sohir l*hinfl.
h)
/
/
'\
>." ^
\
V
"j
#"
/
/
/
""^^
\
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/-;X'U\>,^
:\^
W"'
-\>w-/^
/
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fci
J
UKANU8 AND NEPTUNE.
n
ing the star and the stm, may be at once negatived. The
question is therefore reduced to (!) wliether the place
of the star is vertically above or beneatli the polar axis
of the sun ; or, (2) whether it is above or below the
equatorial plane of the sun ; or (3) whether the place of
the star is in the equatorial plane of the sun. The (1)
case it is not necessary to consider, as such a supposition
is clearly inadmissible ; but to decide astronomically be-
tween the (2) and (3) — that is, whether the place of the
star is above, below, or in the equatorial plane of the sun
— will probably require further careful observation of the
planet. The observations already recorded seem, how-
ever, to support the supposition that the place is consi-
derably above the equatorial plane of the sun, as shown
in the illustrations, plates 14 and 15 ; the conclusion that
such is the true locality of the star will be somewhat
strengthened by including the circumstances at present
ascertained of the still more recently discovered and less
known planet Neptune.
Taking the assumption that Uranus belongs to a neigh-
bouring stellar system, the probability is at once sug-
gested that Neptune is another member of the same sys-
tem and at a less distance from the central body.* The
few observations as yet recorded of this planet cannot be
* On the assumption that Uranus and Neptune belong to the solar system,
the orbital distance between Mercury and Neptune should be, according to
Eode's law, twice that between Mercury and Uranus. It is now estimated
from observation, only to exceed the latter by a little more than half the
distance. ' i '
m
I "■■
22
URANUS AND NEPTUNE.
considered, on account of the great distance of the planet
and difficulties of observing it, as very reliable ; two
moons are reported, of which one has " an orbit " — ac-
cording to Mr. Otto Struve — " inclined to the ecliptic at
the considerable angle of 35° ; but whether, as in the
case of the satellites of Uranus, the direction of its
motion be retrograde, it is not possible to say until it shall
have been longer observed." Now, an angle of 35°
differs considerably from perpendicularity; but, even if
admitting the coiTectness of the observation, we must
remember it was made on an assumption (prejudice) that
the earth and the planet were in the same or nearly in the
same plane ; whereas, if we assume Neptune to belong
to the neighbouring stellar system, that planet would
probably be considerably above the plane of the earth's
orbit, and consequently (as before shown, with regard to
the solar spots, in Part Second) an erroneous inference as
to the obliquity of the satellites' plane of revolution
would be occasioned. See the accompanying figure
(Fig. 1), where the lower body E may be considered to
represent the earth.
Fig. 1.
i
/•/u/f I i-
Til K Til Mom'
O V
TIIK STMLLAR IWIVKHSK.
The 'S' II IIS ./.t/.if
heiutf ronsiiirn't/ VrrlhnI
»\ii
Safuim
Enrfh
S
vn
Earth
K.
-///
Earth
i! i.
If
• Tllh: THEORY
OF
TIIK STIi:LLAIi rNlVERSE.
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yf%^.
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^
^
v^
"4
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I I
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EaHk
S
un
Earth
Sfi/t(,-rt
T/ranus
.Vf/jiiine
iTrnrm^
i
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.;
J
T
y
Ha/r 15
'1' i i I^ 'Y II E ( ) I^ Y
THE STELI.An rNIVf^RSE
I'ritu/ mnsulvn'il Horcovlal
^•!^^^
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^/:^
^//
♦»♦-
:;ii
G.
//
i»
Hff/r 15
Til f: theory
O V
rilJ'] STt:LI,AI[ UN [VERSE.
Tin' -Sim's Arh
I'rhu/ ronnuh'n'd Honzovtal
^-
/i
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m
®
^
r/i
ferh'cal aaris
'■9*-
V
4
/
®
' MATTER AND MATERIAL LAWS. 23
Summing up the consideration of the cas<}, we conclude
(1) that the planets Uranus and Nepttmt are not solar
planets belonging to cur system, but that they are stellar
planets belonging to a neighbouring system which has its
axis of revolution perpendicular to that of the solnr
system ; (2) that the (central) star round which those
planets revolve is very probably above the equatorial plane
of our sun j and (3) that the distance of the star from
the sun may be roughly estimated (by adding the distance
of the planet Saturn to that of Uranus, 890 + 1800 =
8690 million miles) at about 3000 million miles.
(See Plates 14 and 15.)
Note. — The following apparently weighty or fatal olijec-
tios to the opinion expressed above — that the planets Uranu-s
and Neptune belong not to the solar but to a neighbouring
steiiPT system — is very likely to suggest itself at once . —
The planet Uranus has been for a considerable time, nearly
a century, directly uuder astronomical observation, and
occasional notices of its having been previously observed,^
and mistaken for a star, are on record. Now if evidence-
can be shown that the planet has been observed at succes-
sive places in its (alleged) solar orbit, and, so to speak,
tracked throughout its orbit; or if, having been seen atone
extremity of its supposed solar orbit, it has been subse-
quently observed at the opposite extremity, then the
opinion stated by us cannot certainly be upheld. It is
therefore to bo understood that the strong and confident
■ f
\
S(
24
nOMETAUY OUBIT8.
opinion Ktiited inc-liidos tlie opinion on our part that no such
oviiloneo of an nctnal Kolar orltit can bo shown*
(4,) Masses (if agtjreyntcd maffcr uvil tlirir relation to
the laws of the material uniccrsc.
The nssuinption that masses of matter, revolving around
centres of gravitating influence in the iieighbouriioofl of,
but not belonging to the solar system, may approach
sufficiently near to be visible from the earth, will perhaps
enable us to understand and give a reasonable explana-
tion of some of those observed facts of astronomy, which
at present occupy the position of mechanical eflects ap-
parently governed and regulated by laws unknown to or
unrecognized by mechanical science. We allude more par-
ticularly to those very various bodies at present group-
ed and classed together under the name comet. Plate ]
from the (Encyc. Britannica) is an example of the illus-
trationo given at the present time in astronomical works,
of the supposed orbital revolution of a comet around the
sun. In some cases the orbital path is considered to be an
ellipse of extreme eccentricity ; in other cases, a'para-
bola ; or, a hyperbola. The objection to this teaching
seems to have been overlooked that it is inadmissible in
a scientific sense, because contrary to the law of gravi-
tation ; a law which is recognized both by astronomical
* Considered as solar planets, theory assigns 84 years as the period of
Uranus, and about 165 years as that of Neptune.
uch
n to
uiid
lof,
>ach
baps
« na-
il ich
i ap-
;o or
par-
oup-
e IG
Uus-
Drks,
i the
►e an
para-
!hing
tie in
ravi-
nical
iod of
I'LATH (i.
Fimn the Emyv. JJritln.
COMETARY ORBITS. |||
and mechnnicul science. In Fig. 2, the body C, to the
north-east of the sun, isinovingwlth an increasing velocity
in the direction BD. The gravitating influence of the sun
is supposed, at this place in the cornet's orbit, to exceed
the centrifugal force, causing it to gravitate towards and
Fio. 2.
approach the sun. Since the approach is very considerable
in extent and rapid, so is the increase in the velocity pro-
portionately great, and when the comet has arrived (i.e.,
supposed to have arrived) at its perihelion P, it is moving
with enormous velocity past the sun in the direction DE ;
for a certain short distance, it proceeds in a curve not
differing very much from the arc of a circle, but then,
notwithstanding that it is supposed to be comparatively
26
COMET ART ORBITS.
very near the sun and under the influence of an enor-
mous attractive force, it suddenly ceases altogether to
obey this force, and proceeds in the direction EF, as
shown in the figure, without any further regard to the
central gravitating influence. If the body is material
and subject to the known laws governing matter
when moving from B towards D, and if, even after passing
its perihelion, it still retains its material i ature and re-
cognizes the influence of gravitation until beyond E,
how is it to be admitted that its subjection to the laws
of matter can be suddenly abrogated! We cannot
admit a supposition that any material mass, having
once become subject to the sun as the central gravitating
influence governing its motion, and thus belonging
to the solar system, can suddenly throw off" its allegi-
ance and withdraw from the sun's controlling power into •
space, or to visit some other system in a similarly capri-
cious manner. If we assume the body (comet) to have
an'ived at the place (P) shown in the figure, nearest the
sun (without troubling ourselves to explain how it got
there), and to be moving past the sun with such very great
velocity that the centrifugal force developed is more than
sufficient to counterbalance the enormous attractive force
of the sun, at so short a distance ; then the inference will
be sound that the comet must recede from the sun ; and
further, the distance to which the comet will recede will
be proportional to the excess in the centrifugal force
over the gravitative force when nearest to the sun, as
explained and demonstrated in Part First of this Series ;
but even in such case the recession could only take place
in an orbit with a continually increasing radial distance
from the sun, as shown in Fig. 3, and the path of the
DISTANCE OF THE VISIBLE STARS.
21
TecediDg body would have the form of a spiral curve
continually increasing outwards from the sun as its
centre.
Fio. 3.
(5) The relative distance of the visible stars.
Previously to explaining the real character of the
cometary motions, it will be proper to examine the ques-
tion as to the relative distances of the visible stars. In
Plate 6 (Fig. 10), the illustration plainly shows, that
assuming the pole star, for instance, to be at a much
less distance than is attributed to it by astronomers at
tlie present time, the orbital movement of the earth
would not suffice to much alter tlie apparently relative
place of the star. To an observer viewing it from the
earth, it would appear almost directly over the pole — in
whatever part of the orbit the earth's place might be at
the time of the observation. And further, it will be
found that if the assumed distance of the star be again
diminished, and taken at (let us say) one half the dis-
|:1
2S
DISTANCES OF THE VISIBLE STARS.
taiice shown in that illustration, the difference to the-
terrestrial observer would still be difficult to detect with-
out some object having a relatively fixed position to com-
pare with ; for example (assuming the axes of the earth
and sun to be both perpendicular to the plane of revolu-
tion), if the star was directly over the pole of the sun, it
would be extremely difficult, even if the distance of the
star was very much less than is supposed, to detect any
difference ; by careful and precise determination of the
celestial sphere, however, and by comparison with a num-
ber of the (so called) fixed stars, a point could be found
which would be relatively motionless and around which
the earth's pole star would appear to revolve in a small
circle — the direction of the earth's actual revolution be-
ing reversed in that of the apparent revolution of the
star ; and this effect would still be essentially the same
even if the distance was very great, and indeed so long
as the star remained visible ; only that, tlie greater the
distance of the star from the earth, the less would be the
diameter of the circle in which the earth's pole star
would appear to revolve around the point representing
the pole of the celestial sphere ; and if the distance was
extremely great, so would the apparent circle of revolu-
tion be very small. The apparent motion or change in
the apparent position of the pole star, since it would
result from a change in the observer's actual position,
would be astronomically termed the effect of " parallax ;"*
• Parallax may be either geocentric or heliocentric ; in the one, the
diameter, or pnrt of the diameter, of the earth ; iu the other, the diameter
or part of the diameter of the orbital circle of the earth's revolution, is the
measured base of the triangle. The general expression " parallax " is well'
defined, in Lardner's Astronom;;, as " the apparent displacement of any
ob'ect seen at a distance, due to a change of position (place) cf the
observer."
DISTANCES OF THE VISIBLE STARS.
29
and as the distance of the earth from the sun is (approxi-
mately) known and therefore tlie diameter of the earth's
orbit, the distance of the star from the sun (or earth)
could be thus measured by parallax. . . Have the distances
of the various stars been thus ascertained by parallax ?
The question will be answered by the following extracts
from the Astronomical Record.
Note. — We will here again remind the reader that by the perpendicular
axis theory such a parallax of the earth's polar-zenith (pole-star) can be
only attainable by observations n' ie when the earth is passing or repass-
ing the sun's equatorial plar ; if the observations be made at other times,
the parallactic effect of the horizontal movement would be entirely masited
or much interfered with by the effect of the vertical movement. And,
Again, should it appear that the earth is subject, as we have supposed, to a
vibration on a horizontal axis transverse to a line joining the earth and
sun this would constitute an independent interfering cause unless the
observations were made at the nodal plane when the earth's position is
strictly perpc idicular
(11.
Hi
HI
'Vil
[-(
e:zi
* CHAPTER II. V
The present mixed doctrine of Parallax and Aberration,
(6) Theory OF Parallax.
HerscheVs Outlines of Astronomy.
(800.) " The diameter of the eailu haa served us for the
base of a triangle, in the trigonometrical survey of our sys-
tem (art. 274), by which to calculate the distance of the
sun ; but the extreme minuteness of the sun's parallax (art.
275) is so delicate, that nothing but the fortunate combi-
nation of favourable circumstances afforded by the transits
of Venus (art. 479) could render its results even tolerably
worthy of reliance. But the earth's diameter is too small a
base for direct triangulation to the verge even of our own
system (art. 626), and we are, therefore, obliged to substi-
tute the annual parallax for the diurnal, or, which comes
to the same thing, to ground our calculation on the relative
velocities of the earth and planets in their orbits (art. 486),
when we would push our triangulation to that extent. It
might be naturally enough expected that by this enlarge-
ment of our base to the vast diameter of the earth's orbit,
the next step in our survey (art. 275) would be made at a
great advantage ; — that our change of station, from side to
side of it, would produce a considerable and easily measur-
able amount of annual parallax in the stars, and that by its
means we should come to a knowledge of their distance.
But, after exhausting every refinement of observation,
astronomers were, up to a very late period, unable to come
to any positive and coincident conclusion upon this head ;
and the amount of such parallax, even for the nearest fixed
star examined with the requisite attention, remained mixed
up with and coneealed among the errors incidental to all
astronomical determinations. The nature of these errors
JELIOCENTRIO PARALLAX.
31
" has been explained in the earlier part of this work, and we
need not remind the reader of the difficulties which must
necessarily attend the attempt to disentangle an element
not exceeding a few tenths of a second, or at most a whole
second, from the host of uncertainties entailed on the results
of observations by. them : none of them individually, per-
haps, of great magnitude, but embarrassing by their number
and fluctuating amount. Nevertheless, by successive refine-
ments in instrument-making, and by constantly progressive
approximation to the exact knowledge of the Urano-
graphical corrections, that assurance had been obtained,
even in the earlier years of the present century, viz., that
no star visible in northern latitudes, to which attention had
been directed, manifested an amount of parallax exceeding
a single second of arc. It is worth w^^'le to pause for a
moment to consider what conclusions would follow from
the admission of a parallax to this amount."
(801.) " Radius is to the sine of 1" as 206265 to 1. In
this proportion then at least must the distance of the fixed
stars from the sun exceed that of the sun from the earth.
Again, the latter distance, as we have already seen
(art. 357), exceeds the earth's radius in the proportion of
23984 to 1. Taking, therefore, the earth's radius for unity, a
parallax of 1" supposes a distance of 4947059760, or nearly
five thousand millions of such units ; and lastly, to descend
to ordinary standards, since the earth's radius may be taken
at 4000 of our miles, we find 19788239040000, or about
twenty billions of miles for our resulting distance."
(802.) " In such numbers the imagination is lost. The
only mode we have of conceiving such intervals at all is by
the time which it would require for light to traverse them.
(See note §, at the end of this chapter, for a more familiar
illustration.) Light, as we know (art. 545), travels at the
rate of a semidiameter of the earth's orbit in S"- I'd'- '6. It
would, therefore, occupy 206205 times this interval, or
32
HELIOCENTRIC PARALLAX.
"3 yonra and 83 days, to traverse the distance in question.
Now, as this is an inferior limit whicli it is already ascer-
tained that even the brightest and therefore i)robably the
nearest stars excoed, what are we to allow for the distance
of those innumei'able stars of the smaller magnitudes which
the telescope discloses to us ! What for the dimensions of
the galaxy in whose remoter regions, as wo have seen, the
united lustre of myriads of stars is perceptible only in pow-
erful telescopes as a feeble nebulous gleam 1"
(803.) " The space-penetrating power of a telescope, or
the comparative distance to which a given star would
require to bo removed in order that it may appear of the
same brightness in the telescope as before to the naked oyo,
may be calculated from the aperture of the telescope com-
pared with that of the pupil of the eye, and froi;>. 5ts reflect-
ing or transmiting power, i.e. the proportion of the incident
light it conveys to the observer's eye. Thus it has been
computed that the space-penetrating power of such a reflec-
tor as that used in the star-gauges above referred to is
expressed by the number *75. A star, then, of the sixth
magnitude removed to 75 times the distance would still be
perceptible as a star with that instrument, and admitting
such a star to have 100th part of the light of a standard
star of the first magnitude, it will follow that such a stand-
ard star, if removed to 750 times its distance, would excite
in the eye, when viewed through the gauging telescope, the
same impression as a star of the sixth magnitude does to
the naked eye. Among the infinite multitude of such stars
in the remoter regions of the galaxy, it is but fair to con-
clude that innumerable individuals, equal in intrinsic bright-
ness to those which immediately surround us, must exist.
The light of such stars, then, must have occupied uj^wards
of 2000 years in travelling over the distance which sepa-
rates them i'rora our own system. It follows, then, that
when we observe the places and note the ajjoearances of
HELIOCENTRIC PARALLAX.
33
<' such stars, we are only reading their history of two thousand
years anterior date, thus wonderfully recorded. We cannot
escape this conclusion but by adopting as an aU-^rnative an
intrinsic inferiority of light in all the smaller stars of the
galaxy. We shall be better able to estimate the probability
of this alternative when we have made acquaintance with
other sidereal systems whose existence the telescope dis-
closes to vis, and whose analogy will satisf}- us that the
view of tl.e subject here taken is in perfect harmony with
the general tenor of astronomical facts."'
(SOi.) " Hitherto we have spoken of a parallax of 1" as a
mere limit below which that of any star j'et examined
assuredly, or at least very probably falls, and it is not with-
out a ceitain convenience to regard this amount of parallax
418 a sort of unit of reference, which, connected in Iho
reader's recollection with a parallactic unit of distance from
our system of 20 billions of miles, and with a H^ ^-ears'
journey of light, may save him the trouble of such calcu-
lations, and ourselves the necessity of covering our pages
with such enormous numbers, when speaking of stars whose
parallax has actually been ascertained with some approach
to certainty, either by direct meridian observation or by
more refined and delicate methods. These we shall proceed
to explain, after first pointing out the theoretical peculiar-
ities which enable us to separate and disentangle its etiects
from those of the Urano-graphical corrections, and from
•others causes of error which, being periodical in their nature
add greatly to the difficulty of the subject. The effects of
precession and proper motion (see art. 852), which are uni-
formly progressive from year to year, and that of nutation
which runs through its period in nineteen year^, it is
obvious enough, separatej^themselves at once by these
<!haracters from that of parallax ; and, being known with
very great precision, and being certainly independentj' as
regards their causes, of any individual peculiarity in the
1-
34
PARALLAX OF THE STARS.
" stars affected by them, whatever small uncertainty may
remain respecting the numerical elements which enter into
their computation (or in mathematical language their co-
eflScionts), can give rise to no embarrassment. With regard*
to aberration, the case is materially different. This cor-
rection affects the place of a star by a fluctuation, annual in
its period, and therefore, so far, agreeing with parallax. It
is also very similar in the law of its variation at different
seasons of the year, parallax having for. its apex (see art.
343, 344.) the apparent place of the sun in the ecliptic, and
aberration, a point in the same great circle 90" behind that
place,^ so that in fact the forraulea of calculation (the co-
efficients excepted) are the same for both, substituting only
for the sun's longitude in the expression for the one, that
longitude diminished by 90° for the other. Jloieovor, in
the absence of a6so/Mie certainty respecting thti nature of the
propagation of light, astronomers have hithoi'to considered
it necessary to assume at least as a possibility that the
velocity of light may bo to some slight amount dependent
on individual peculiarities in the body emitting it."*
(805.) " If we supjjose a line drawn from the star to the
earth at all seasons of the year, it is evident that this line
will sweep over the surface of an exceedingly acute, oblique
cone, having for its axis the lino joining the sun and star,
and for its tase the earth's annual orbit, which, for t..>>
pi'esent purpose, we may suppose circular. The star will
therefore appear to describe each year about its mean place
regaixled as fixed, and in virtue of parallax alone, a minute
ellipse, the section of this cone by the surface of the celes-
tial sphere, perpendicular to the visual ray. But there is
* " In the actual state of astronomy and photology, this necessity can
hardly be considered as still existing, and it is desirable, therefore, that the
practice of astronomers of introducing an unknown correction for the con-
stan^f aberration into their equatians of condition for the determination
of pWallaz, should be disused, since it actually tends to introduce error
into the fiaal result."
PARALLAX AND ABERRATION.
35-
" also another way in which the same fact may be repre-
sented. The apparent orbit of the star about its mean place^
as a centre, will be precisely that which it would appear to
describe if seen from the sun, supposing it really revolved
about that place in a circle exactly equal to the earth's
annual orbit, in a plane parallel tc» the ecliptic. This is
evident fi-om the equality and parallelism of the lines and
directions concerned. Now, the effect of aberration (disre-
garding the slight variation of the earth's velocity in
different parts of its orbit) is precisely similar in law, and
differs only in amount, and in its bearing reference to a
direction llO" different in longitude. Suppose, in order to
fix our ideas, the maximum of parallax to be 1" and that of
aberration 20-5", and let AB, ab, bo two circles ima^^ined
to be described separately, as above, by the star about its
mean place S, in virtue of these two causes respectively,
St being a line parallel to that of the line of equinoxes.
Then if, in virtue of pr-rallnx alone, the star would bo found
at a, in the smaller orbit, it would, in virtue of aberration
alone, be found at A in the larger, the angle, a S A, being
a right angle. Drawing then A C equal and parallel to S a,
and joining S C, it will, in virtue of both simultaneously, bo
found in C — i. e. in the circumference of a circle whose
radius is SC, and at a point in that circle in advance of A
,!
36
PARALLAX AND ABERRATION.
" the nborrational place, by tho angle ASC. Now, since
SA : AC : ; 20-6 ; 1, wo find, for tho angle ASC, 2° 47' 35";
and for the length of the radius SC, of the circle represent-
ing tho compound motion 20".524. The difference (0".024)
between this and SC, the radius of the aberration circle, is
quite imperceptible, and even supposing a quantity so
minute to be capable of detection by a prolonged series of
ooservations, it would remain a question whether it were
produced by parallax or by a specific difference of aberration
from the general average 20".5 in the star itself. It is,
therefore, to the difference of 2° 48' between the angular
situation of tho displaced star in this hypothetical orbit, i, e.
in tho arguments (as they ai'O called) of the joint correction
(ySC) and that of abberration alone (ySA), that we have to
look for the resolution of the problem of parallax. Tho
reader may easily figure to himself tho delicacy of an
enquiry which turns wholly (even when stripped of all its
other difficulties) on the p'eme determination of a quantity
of this nature, and of such very moderate magnitude."
The form of the figure illustrating the case defines the
relative position which the observer's station is supposed
to occupy, for, because A.B., a.h. are circles described
by the star about its mean place S., a line passing
through the star and through the earth must be perpen-
dicular to the plane of the circles in every direction, or
in other words, it must be a transverse axis to the circles
passing perpendicularly through their common centre.
Now parallax is an effect consequent ujwn an alteration
in the observer's position, and parallax of the fixed stars
is an effect consequent upon the constantly progres.sive
change of the earth's place in its orbit. The parallactic
circle of the star's apparent movement in the heavens
is the representation, the inverted reflection, of the
earth's actual movement in its orbital revolution. There-
PARALLAX AND ABERRATION.
37
fore the parallactic displacement of the star through the
semi-diameter of the circle, from Stoals consequent upon
the (completed) motion ot the earth through its second
semi-orbit in the opposite direction, viz : correspondent to
the semi-diameter SJ. For if a be the extreme east of
the star's parallactic circle, it is tlie iipparent place of the
star observed from the earth wlicn at the western
extremity of its orbit, and tlie stir's apparent motion
from S. to «. is the gradually increasing effect of the
eartli's motion from the central place of its orbit to the
extreme west thereof. Aberration is an (hypothetical)
effect consequent upon the motion of tlie person whose
eye receives the light from the object, and aberration of
the hxed stars is a (supposed) apparent effect conse-
quent upon the actual motion of the eartli in its orbit.
The supposed aberrational circle of the star's apparent
motion in the heavens is a representation or reflection
of the vdocity and I'he direction of the earth's orbital
motion. Therefore the aberrational displacement of the
star consequent upon the earth's orbital motion in the
direction D. S. will be in the opposite direction, viz:
from A. towards the west (i- e. the opposite direction to
A. c.) For the aberrational displacement of the star to
take effect in the direction S. A, the earth's orbital
motion must evidently be in the direction opposite
thereto, viz : in the direction A. S. or A.B. / but oi-bital
motion of the earth in such direction cannot cause paral-
lactic displacement in the direction S. a, nor yet in the
direction a. S, bocause the displacement belonging to
parallax as well as the supposed displacement due to
aberration must be parallel to the motion or altera-
tion in relative position of the observer's station, upon
38
I'AHAI-l.AX AND AUEURATION.
wliich «'.ich is dnpeiuUMit and of which both • are
coiisiMii.ents.
W»? are, thoreforf, quite iitsable to accept the state-
ment, here made and deiiiied by illustration, of an
a)t|>arent theoretical motion by the star, compounded of
an aberrational effect at right angles to the effect of
parallax. In order that the reader may be able to fairly
consider the evidence in this and other cases, we will
presently give as fully and completely as our limits per-
mit. Sir John Herschel's own statement and definition of
the doctrine of aberration.
If aberration be indeed a reality, if it be anything
more than a chimera of the imagination, some intelligible
reason consistent with the theory to whioli it belongs can
be shown why aberrational displacement should take
effect in a direction at right angles to that of the dis-
placement due to parallax.
Assuming, for a moment, the aberrational eflfect to be
a reality in its application to the case here illustrated,
that effect must be so related to that of parallax that the
one is a deduction from the other, because when the one
causes, or tends to cause, an effect in the one direction,
the other causes, or tends to cause, an effect in the con-
trary direction, so that if the respective effects of the
two causes should be exactly equal, the one must neu-
tralize the other, and no apparent displacement of the
star would take place.
The characteristic difference between the two effects
is. that of parallax is consequent upon a completed
movement from one place to another more or less distant
from the first : in the case of the earth's orbital motion
it is progressive from any given place in the orbit as a
PARALLAX AND ABRHRATION.
89
starting point until the fintirc diiimetcr of the orbit lias
Iteen completed. The qiuintity of oppurciit eflecton the
stiir, for any given distunce moved through by the enrth,
is dependent upon the diHtnnce of the stur ; the result is
<piite independent of the velocity of tiie earth's motion
(unless indirectly as effecting the distance moved
through in a given time) ; whatever distance has been
actually moved tlirough by the earth there must be a
proportional optical eff«'ct of parallax, i. c, an apparent
displacement, although if the distance moved through by
the earth be comparatively small and the distance of the
star very great, such effect may be too minute to be appre-
ciable by the terrestrial observer, and it is indeed barely
conceivable by the mind that the distance of a star
might be so enormously great that the parallactic effect
of the earth's motion, from one extremity of its orbit to
the opposite, would be less than instrumental astronomy
in its present state is able to take cognizance of.
According to the theory of aberration, the supposed
effect thereof, which is wholly dynamical, is dependent
upon the motion of tlie observer, whose eye receives the
light, relatively to the obsen-ed object, and the amount
of the effect dependent upon the velocity of that motion,
consequently, if the earth be supposed to move from the
extreme west to the extreme east of its orbit, the
maximum aberrational displacement of the star towards
the east will be attained at the time the earth passes the
place in the orbit half distant between the east and
west ; but the displacement will be nearly as great for
a considerable time before the earth arrives at the half-
distance, and will begin to diminish so soon as the place
of half distance has been passed ; whereas the maxi-
f^
40 INTERFERING DIPFICCLTIES.
muin of pinallactic effect is only attained when the earth
has arrivefl at the opposite extremity c." the orbit, at
which time tlie supposed aberrational .jttect due to the
motion of the earth in tliat direction will have wholly
ceased. It is true that by the theory of aberration there
would be now a displacement, at right angles to the
former, due to the earth's orbital motion from north to
souili, but this would be in the opposite direction to that
shown in Ilerschel's illustration, and it would be coun-
terar<-cJ by tlie parallactic eftect also belonging to the
earth's motion from north to south ; and, be it observed,
this last effect, even according to the aberration theory,,
could only take place in the case of a polar star (to
which alone Herschel's illustration applies), for, in the
case of an equatorial star, since the earth would be
moving directly away from the star, no aberrational (or
parallactic) effect whatever could tfike place as a con-
sequent to the earth's motion in that direction, viz.,
from north to south.
The quotation from HerscheVs Outlhicft in Astronomif
continued.
(806.) " But these other difficulties themsclvos are of no
trifling order. All astronomical instruments are affected
hy difl'erencos of temperature. Not only do the materials
of which they are composed expand and contract, but the
masonry and solid piers on which they are erected, nay
even the very soil on which those are founded, participate
in the general change from summer warmth to winter cold.
Hence arise slow oscillatory movements of exceed injrly
minute amount, which levels and plumb lines affoi-d but
very inadequate means of detecting, and which, being aha
annual in their period (after rejecting whatever is casual and
momentary), mix themselves intimately with the matter
PARALLAX OF THE STARS.
4i
<'of our enquiry. Refraction too, besides its casual vari-
ations from night to night, which a long series of observations
would eliminate, depends for its theoretical expression on
the constitution of the strata of our atmosphere, and the law
of the distribution of heat and moisture at different eleva-
tions, which cannot be unaffected bj' difference of season.
No wonder, then, that mere meridional observations should,
almost up to the present time, have proved insufficient
except in one very remarkable instance, to afford unques-
tionable evidence, and satisfactory quantitative measurement
of the parallax of any fixed star."
(807.) " The instance referred to is that of a Centauri, one
of the brightest and, for many otlicr reasons, one of the
most remarkable of the southern stars. From a series of
observations of this star, made at tlie Royal 01).scrvator3- of
the Cape of Good Hope in the years 1832 and 1833, by
Professor Henderson, with the mural circle of that estab-
lishment, a parallax to the amount of an entire second was
concluded on his j-cduction of the obsei'vations in question
after his return to Khghind. Subsequent observations by
^Iv. Maclear, partly' with the same, and partly with a new
and far more effici >ntly constructed instrument of tlie same
description mad', in the years 1839 and 1S40, have fully
loiitirmed the reality of the parallax indicated b}- Professor
Henderson's observations, though with a slight iliniinution
ill tU condud^jd amount, which comes out equal to 0"-9128
or alxMit jytlis of a second ; Irujht stars in its immediate
uei(/hboiirk^x/d being unaffected by a similar periodical dis-
placement, and tlius affording satisfactory proof that the dis-
jdacement indicated in the case of the star in question is )iot
merely a result of annual variations of temperature. As it
is im^wssible at present to answer for so minute a quantity
as that by which this result differs from an exact second,
wo may consider the distance of this star as. approximately
48
PARALLAX OF THE STARS.
"expressed by the parallactic unit of distance referred 1o irt
art. 804."
(808.) " A short time previous to the publication of this
important result, the detection of a sensible and measurable
amount of parallax in the star No. 61 Cygni of Flamsteed's
Catalogue of Stars, was announced by the celebrated astron-
omer of Konigsberg, the late M. Bessel. This is a small
and inconspicuous star, hardl}' exceeding the sixth magni-
tude, but which had been pointed out for especial obser-
vation by the remarkable circumstance of its being affected
by a proper motion (sec art. 852), i.e. a regular and contin-
ually progressive annual displacement among the sur-
rounding stars to the extent of more than 5" per annum, a
quantity so very much exceeding the average of similar
minute annual displacements which many other stars
exhibit, as to lead to a suspicion of its being actually nearer
to our system. It is not a little remarkable that a similar
presumption of proximity exists also in the case of a Cen-
tauri, whose unusually large proper motion of nearly 4" per
annum is stated by Profess'>r Henderson to have been the
motive which induced him to subject his observations of
that star to that severe discussion which led to the detection
of its parallax. M. Besscl's observations of tJl Cygni were
commenced in August, 1837, immediately on the establish-
ment at the Kiinigsberg observatory of a magnificent heli-
ometer, the workmanship of the celebrated optician Fraun-
hofer, of Munich, an instrument especially fitted for the
system of observation adopted ; which, being totally dif-
ferent from that of direct nu-ridional observation, more
refimed in its conception, and suscejitible of far greater
accuracy in its practical application, we must now explain."
(809.) "Parallax, i»ro])er motion, and specific aberration
(denoting by the latter piirasc that pai't ol the aberration of
u star's light which may be supposed to ari'^e from its indi-
vidual peculiarities, and which we have every reufcou to
PARALLAX OF STARS OPTICALLY DOrBLE.
43
" believe, at all events, an exceedingly minute fraction of the
\7hole), are the only uranographieal corrections which do
not necessarily affect alike the apparent places of two stars
situated in, or very nearly in, the same visual line. Sup-
posing, then, two stars at an immense distance, the one
behind the other, but otherwise so situated as to appear
very nearly along the same vi.-ual line, they will constitute
what is called a star optically double, to distinguish it from
a star physically double, of which more hereafter. Aber-
ration (that which is common to all stars), precession,
nutation, nay, even refraction, and instrumental causes of
apparent displacement, will affect them alike, or so very
nearly alike (if the minute difference of their apparent
places be taken into account), as to admit of the difference
being neglected, or very accurately allowed for, by an easy
calculation. If then, instead of attempting to determine by
observation the place of the nearer of two very unequal stars ■
(which will probably be the larger) by direct observation of
its right ascension and polar distance, we content ourselves
with referring its place to that of its remoter and smaller
companion by differential observation, i.e. by measuring only
its difference of situation from the latter, we are at once
relieved of the necessity of making these corrections, and
from all uncertainty as to their influence on the result.
And for the very same reason, errors of adjustment (art.
736), of graduation, and a host of instrumental errors,
which would, fur this delicate purpose, fatally affect the
absolute determination of either star's piaeo, are harmless
when only the ditfei*ence of their places, each equally
affected by .such causes, is required to bo known."
(810.) "Throwing aside, therefore, the consideration of
all these errors and corrections, and tlisregai-iling for the
present the minute efl'ect of aberration and the uniformly
])rogre8sive effect of proper motion, let us trace the effect of
the difforenccs of the parallaxes of two stars thus juxta-
^'
I
44
PARALLAX OP STARS OPTICALLY DOUBLE.
i
" posed, or thoir apparent relative distance and position at
various seasons of the year. Now, the parallax being
inversely as the distance, the dimensions of the small
ellipses appai'cntlj' described (art. 805) by each star on the
concave surface of the heavens by parallactic displacement
will differ — the nearer star describing the larger ellipse.
But both stars lying verj' nearly in the same direction from
the sun, these ellipses will be similar and similarly situated.
Suppose S and s to bo the positions of the two stars as seen
from the sun, and let ABCD, abed, be their parallactic
ellipses; then, since they will ■
be at all times similarly situ-
ated in the^o ellipses, when the
one etar is seen at A, the other
will be seen at a. When the
earth has made a quarter of a ^
revolution in its orbit, their
apparent places will be 36 ; ^
when another, quarter, Cc ; and
when another, Dd. If then,
Ave measure carefully, with mi-
crometers atlapted for the purj^oscs, their apparent situation
with respect to each other, at different times of the year,
we should perceive a periodical change, both in the direction
of the line joining them, and in the distance between their
centres, For the lines A't, and Cc, cannot be parallel, nor
the lines B6, and Dd, equal, unless the ellii)ses bo of equal
dimensions, i.e. unless the two stars have the same parallax,
or are equidistant from the earth."
In examining the case here illustrated we are in u3ubt
as to the latitudinal place of the star represented in the
figure and with respect to which no information appears
to be given. It must be remembered that Herschei
assigns to the earth an orbit horizontal to the axis of the
PARALLAX OF STARS OPTICALLY DOUBLE.
45
celestial sphere; consequently, it seems to us that a paral-
lactic ellipse, having its major axis perpendicular to the
station of the terrestrial observer, as shown in the figure,
could be only obtained by positing the celestial sphere
horizontally. Let us suppose the double star under exami-
nation to be Polaris with a companion star actually at a
much greater distance but apparently in close proximity.
We then have Herschel's figure modified as in fig. ^ . .
because the sum of the annual parallactic movements
(displacements) of the two stars would be circles reflect-
ing the orbital revolution of the earth, the greater circle
belonging to the nearer star, the lesser to the more dis-
tant.* Now, if we suppose the star to be equatorial, or
nearly so, we shall have the figure modified as shown at
C. . . that is, the two stars would appear to shift their
position almost linearly, having a reciprocating move-
ment to and fro annually, in the same line, or in an
extremely elongated ellipse. Again, if we suppose the
double star to be located intermediately between the
celestial pole and equator, we then have an ellipse such
as shown oX B.
I :
\ '<:
* From this figure it may be readily understood that the phenomenon of
a revolving double star may be occasioned by the greater parallactic dis-.
placement of tlie nearer of two polf.r str.rs which are nearly in the same
u
PARALLAX OF DOCBLE STARS.
i
i
"We have given reasons in the preceding part of this
work why the earth's orbit should be considered to be
compounded of vertical motion as well as of horizontal ;
adopting this theory (of the earth's perpendicular axis)
we shall then have the parallactic effect on the equatorial
star, illustrated by the fig. B instead of by the fig. C,
and in the case of the intermediately located star the
ellipse of the fig. B would be converted into an ellipse
similar to that of Herschel's figure, but placed horizon-
tally instead of upright.*
The quotation from HerschcVs Outlines of Astronomy
continued.
(811.) "Now, micrometers, properly mounted, enable us
to measure very exactly both the distance between two
objects which can bo seen together in the same field of a
telescope, and the position oi the line joining them with
respect to the horizon, or the meridian, or any other deter-
minate direction in the heavens. The double image micro-
meter, and especially the heliometer (art. 200, 2U1), is
peculiarly adapted for this purpo^se. The images of the two
stars formed side by side, or in the same line prolonged,
however momentarily displaced by temporary refraction or
instrumental tremor, move together, preserving their rela-
tive situation, the judgment of which is in no way disturbed
by such irregular movements. The heliometer also, taking
visual line from the earth. That one of the two, which is much the nearer,
appearing to revolve ecrdntricallj once in the year around the more
distant.
• The reader will understand the theoretical representation ot the case
belonging toeach of the respective localities will be thus modified by the
parallactic effect due to the vertical motion of the earth through 4'"
(or 4S°) equalling about 74 million miles, and, therefore, not very much
less than one half the horizontal diameter of the orbit.
PARALLAX OF STARS OPTICALLY DOUBLE.
47
;his
be
tal;
[xis)
)rial
C,
the
lipse
Izon-
in a greater range than ordinary micrometers, enables us to
•compare one large star with more than one adjacent small
one, and to select such of the latter among many near it, as
shall be most favourably situated for the detection of any
motion of the large one, not participated in by its neighbour."
(812.) " The star examined by Bessel has two such neigh-
' hours both very minute, and therefore, probably, very dis-
tant, most favourably situated, the one (s) at a distance of
7' 42", the other (s') at 11' 46" from the large star, and so
■ situated, that their directions from that star make nearly a
right angle with each other. The effect of parallax, there-
fore, would necessarily cause the two distances (Ss, and Ss')
to vary so as to attain their maximum and minimum values
alternately at throe-monthly intervals, and this is what was
actually observed to take place, the one distance being
always most rapidly on the increase or decrease when the
other was stationary (the uniform effect of proper motion
being understood, of course, to be always duly accounted
for). This alternation, though so small in amount as to
indicate, as a final result, a parallax, or rather a difference
of parallaxes between the large and small stars, of hardly
more than one-third of a second, was maintained with such
regularity as to leave no room for reasonable doubt as to its
cause ; and having been confirmed by the further continu-
ance of these observations, and quite recently by the
•exact coincidence between the result thus obtained and that
deduced by M. Peters from observations of the same star at
the observatory of Pulkova, is considered on all hands a?
fully established. The parallax of this star, finally resulting
from Bessel's observation, is 0".348, so that its distance from
our system is very nearly three parallactic units (art. 804)."
(813.) " The bright star a Lyrse has also near it, at only
43" distance (and, therefore, within reach of the parallel
Tvire or ordinary double-image micrometer), a very minute
I
48
PARALLAX OF STARS OPTICALLY DOUBLE.
star which has been subjected, since 1835, to a severe and
assiduous scrutiny by M. Struve, on the same principle of
differential observation. He has thus established the exist-
ence of a measurable amount of parallax in the large star,
loss indeed thar that of 61 Cygni (being only about J of a
second), but yet sufficient (such was the do\icacy of his
measurements) to justify this excellent observer in an-
nouncing the result as at least highly probable, on the
strength of only five nights' observation, in 18.35 and 1836.
This probability, the continuation of the measures to the-
end of 1838 and the corroborative, though not, in this case,
precisely coincident result of Mr. Peters' investigations, have
converted into a certainty. M. Struve has the merit of
being the first to bring into practical application this method
of observation, which, though proposed for the purpose,
and its groat advantages pointed out by Sir William
Herschel so early as 1781, remained long unproductive of
any result, owing partly to the imperfection of micrometers
for the measurement of distance, and partly to a reason
which we shall presently have occasion to refer to."
(814.) " If the component individuals S, s (fig. art. 810) bo
(as is often the case) very close to each other, the paral-
lactic variation of their anyle of position, or the extreme
angle iLcluded between the lines Aa, Cc, may be very con-
siderable, even for a small amount ofdifferenco of parallaxes
between the large and small stars. For instance, in the
case of two adjacent stars 15" asunder, and otherwise
favourably situated for observation, an annual fluctuation
to and fro in the apparent direction of their line of junc-
tion to the extent of half a degree (a quantity which could
not escape notice in the means of numerous and careful
measurements), would correspond to a difference of parallax
of only J of a second. A difference of 1" between two stars
apparently situated at b" distance might cause an oscillatioit
THEORY OF ABERRATION. 49
in that line to the extent of no less than 11°, and if nearer,
one proportionally still greater. This mode of observation
has been applied to a vonsideruble number of stars by Lord
Wrottesley, and with such aa amount of success, as to make
its further application desirable. (Phil. Trans., 1851.)" f
(816.) "Che following are some of the principal fixed
stars to which parallax has been, up to the present time,
more or less probably assigned :
a Centauri 0".976 (Henderson, con-'d by Peters.)
61 Cygni 0".348 (Bessel.)
21258 Laland... 0".260 (Kriiger.)
1U\5-G Oeltzen. 0".247 (Kriiger.)
* a Lym 0".155 (W. Struvo, corr. by O. Struve.)
Sirius 0".150 (Henderson, corr. by Pctern.)
lO.pOphinchi 0".16 (Kruger.)
Ursoe Majoris... 0".133 (Peters.)
Arcturus 0".127 do
Polaris 0".067 do
Capella 0".046 do
• Qy. 0'.255 (see art. 813).
Although the extreme minuteness of the last four of these
results deprives them of much numerical reliance, it is at
least certain that the parallaxes by no means follow the
order of magnitudes ; and this is farther shown by the fact
that a Cygni, one of M. Peters' stars, shows absolutely no
indication of any measurable parallax whatever."
(7.) The Theory of Aberration.
, Let us now examine the " theory of aberration, to
which (as shown by the foregoing quotation) so great an
importance is attached by astronomers.
t See Note on last page (page 89.)
< ■
\
.i
60 TnEORT or aberration.
We will first take the explai)atiori given by Dr.
LarJner.
Handbook of Astronomy.
(2440.) " Aherradon of Zljf//^— Assuming, tl\pn, the velo-
city of light, and th.it the earth is in motion in an orbit
round the sun with a velocity of about 19 miles per second,
which must be its speed if it move at all, as will hereafter
. /pear, an effect would be produced upon the apparent
placcHofall celestial objects, by the combination of these
two motions which we shall now explain.
" It has been statod that tli« apparent direction of a
visible object is in the direction from which the visual ray
enters the eye. Now, this diroetion will depend on the
actual direction of the ray if the eye which receives it be
quiescent; but if the eye bo in motion, the same effect is
produced upon the organ of sense, as if the ray, besides the
motion which is proper to it, had another motion equal and
contrary to that of the eye. Thus, if light moving from
the north to the south with a velocity of 192,000 miles per
second, be struck by an eyo moving from west to oast with
the same velocity, the effect produced by the light upon the
organ will be the same as if the eye, being at rest, were
struck by the light having a motion compounded of two
equal motions, one from north to south, and the other from
east to west. The direction of this compound effect would,
by the principles of the composition of motion (176), be
equivalent to a motion from the direction of the north-east.
The object from which the light comes would, therefore,
be apparently displaced, and would be seen at a point
beyond that which it really occupies in the direction in
which the eye of the observer is moved. This displacement
itt called accordingly the aberration of lioht.
THBORT OP ABERRATION.
It
fr.
Mo-
Jit
Id,
for
^nt
Jse
" This may bo mada Htill more evident by the following
mode of illustration. Let (Fig, 717) l>o the oijject from
^> Q which light comt's in the direction Ooe". Lot fi
\ i he tho place of the eye of the observer when the
\ \ light is at 0, and let the eye bo upposod to
\ j move from e to e", in tho same time that tho
\ i light moves from o to e". Lot a straight tube
\ \ be imagined to bo directed from tho oyo at<^, to
\ i the light at o, so that the light shall bo in tho
• ; centre of its opening, while the tube moves with
> i the eyo from o etooV, maintaining constantly
.'' the same direction, and remaining parallel to
itself: the light in moving from o to e" will pass
along it>< axis, and will arrive af e" when tho oyo
arrives at iliat point. Now it is evident that in
this case tho direction in which the object would
be visible would be tho direction of tho axis of
• the tuV>o, so that, instead of appearing in tho
direction ' -vhich is its true direction, it
•would appear in the direci.. >0', advanceJ from o. in dircc-
tioiiof tho motion e e", with which the i)l>*orvcr is aifected,
"The motion of light being at the rate of 192,000 miles
per second, and that of tho earth (if it move at all) at tho
rate of 19 miles por second (both those velocities will bo
ostablishod hereafter), it follows that the proportion of o e"
to e e" must bo 192,00o to 19, or 10,000 to 1.
" The angle of aberration OoO' will vary with the obliquity
of tho direction e e" of tho observer's motion to that of tho
visual ray o e". In all cases the ratio of o e" to e e" will bo
10,000 to 1. If tho direction of the earth's motion be at
right angles to tho direction o e" of the object 0, we shall
have (2294) tho aberration a = '^^^ « 20".42. If the angle
o e" e be oblique, it will be necessary to reduce e e" to its
component at right angles to o e", which is done by multi-
plying it by the trigonomotriciii sine of the obliquity oe" e of
I
it
«
52
THEORY OV ABERRATION.
" tho direction of tho object to that of the earth's motion. If
thJH obliquity bo exprossoil by 0, wo hHrII hnvo for tho
uliorrations in gcnornl a = 20". 42 x »in. 0, Acconling to this,
the attorrntion would bo groutoiitt when tho direction of tho
earth'H motion \n at right angles to that of tho object, and
would docrottHO as tho angle docroaHOs, being nothing
when tho object in iseon in tho direction in which tho earth
is moving, or in exactly tho controry direction.
" Tho phonomuiia may alsso bo imagined by considering
that the earth, in revolving round tho sun, constantly
chango.s tho direction of its motion ; that direction making
a complete revolution with tho earth, it follows that tho
ctfect produced upon tho ai»parent place of a distant object
would be the name if that object really revolved onco a year
roun<l its true pluco, in a circle whoso piano would bo
parallel to that of tho earth's orbit, and whose radius would
subtend at tho earth an angle of 20".42, and tho object
would bo always seen in such a circle 90' in advance of tho
earth's place in its orbit."
.Since the subject is of great importivnce, we will also
quote the explanation given by Sir John Hcrscliel.
Outlines of Astronomy (page 210) :
(328.) "Neither procession nor nutation change tho
apparent places of celestial oljjocts inter se. Wo see them,
so far as these causes go, as they are, though from a station
more or less unstable, as we soo distant land objects cor-
rectly form 'I I, though appearing to rise and fall whei»
viewed from the heaving dock of a ship in tho act of pitch-
ing and rolling. But there is an optical cause, independent
of refraction or of perspective, which displaces them one
amony the other, and causes us to view the heavens under
an aspect always, to a certain extent, false; and whoso
influence must be estimated and allowed for before we can
obtain a precise knowledge of the place of any object. This
THEORY OF AnKRRATION. |§
"' cauMO is what Ih cal let! tiio ahurrntion uf li^ht ; aHiiigular
and Hurprising ortoct arising from tliiw, that wo occuj))' a
station not at rest, hut in rapid motion ; and tliat tlio appa-
rent diroctionH of tlio rny» of light aro not the namo to a
spectator in motion as to one at rest. Ah tho estimation of
itH effect holongs to uranography, we must explain it lioro,
tliough, in HO doing, wo must anticipate some of tlie results
to ho dotaile<l in Buiweciuont chapters."
(329.) "Suppose a shower of rain to fall perpendicularly
in n dead calm; a person c.\'iiosc<l to the shower, who
should stand quite still and upright, would receive tlie drojis
on his hat, which would thus shelter hint ; but if he ran
Hu'ward in any direction, thej' would strike iiim in tho face.
Tho etl'oct would ho the same as if ho remaine<l still, and a
■wind should arise of the same velocity, and drift them
;igfrinst him. Suppose a Imll let fiijl from a i)oint A, above
w horizontal line EF, and at B wore placed to receive it tho
open mouth of an inclined hollow tube PQ ; if tho tube
Avero Ijeld immoveable, tho ball would strike on its lower
side ; but if the tube were carried forward in the direction
KF, with a velocity projicrly adju.sted at every instant to
that of tho ball, wiiile j)rcserving its inclination to the
horizon, so that when tho ball in sts natural descent reached
C, tho tube should have been carried into the position RS>
it is evident that the ball would, throughout its whole do-
54
THEORY OP ABERRATION.
" scent, be foutul in the axis of the tube ; and a spectator
referring to the tube the motion of the ball, and carrietl
along with the former unconscious of its motion, would
fancy that the ball had been moving in the inclined direc-
tion KS of the tube's axis."
(330 ) . " Our eyes and telescopes are such tubes. In
whatever manner we consider light, wiiether as an advanc-
ing wave in a motionless ether, or a shower of atoms travers-
ing space (provided that m both cases we regard it as
absolutely incapable of suffering resistance or corporeal
obstruction from the particles of tran-ipnrent media traversed
by it*), if in the interval between the rays traversing the
object glass of the one or the cornea of the other (at xohich
moment they acquire that convergence which directs them
to a certain point in fixed space), and their arrival at their
locus, the cross wires of the one or the retina of the other
be slipped aside, the point of convergence (which remains
unchanged) will no longer correspond to the intersection of
the wires or the central point of our visual area. The object
then will appear displaced ; and the amount of the displaoe-
ment is abirration."
(331.) " The earth is moving through space with a velocity
of about 19 miles per second, in an elliptic path round the
sun, and is therefore changing the direction of its motion at
every instant. Light travels with a velocity of 192,000
miles per second, which, although much greater than that
of the earth, is yet not infinitely so. Time is occupied by
* " Tliis conditiun is indispensable. Without it we fall into all
those difficulties which .M. Doppler has so well pointed out in his
paper on Aberration. If light itself, or the luminlferous ether, be
corporeal, the condition insisted on amounts to a formal surrender ol
the dogma, eitl.or of the extension or of the impenetrability of mat-
ter; at least in the sense iu which those terms have been hitherto
used by metaphysicians. At the poin'. to which science is arrived,
probably few will be found disposed to mention either the one or tha
olhor."
THEORY OF ABERRATION.
5»
"it in traversing any space, and in tliat time the earth de-
BcribeB a space which is to the former an 19 to 192,000, or
as the tangent of 20",5 to radius. Suppose now APS, to
represent a ray of light from a -star at A, and let the tube
PQ bo that of a telescope so inclined foi-wai-d that the focus
formed by its object glass shall be received upon its cross
wire, it is evident from what has been said, that the incli-
nation of the tube must bo such as to make PS: SQ::
velocity of light: velocity of the earth : : 1 : tan. 20".6; and,
therefore, the angle SPQ, or PSll, by which the axis of the
telescope must deviate from tiie true direction of the staiv,
must Iie20".5."
(332.) "A similar reasoning will hold good when the
direction of the earth's motion is not perpendicular to the
visual ray. If SB be the true direction of the visual ray.
and AC tlie position in which the telescope requires to be
held in the apparent direction, we mu;<t still have the pro-
l)orlion BC : BA : : velocity of light ; velocity of the eartU.
: : radius : sine of 20".5 (for in such
small angles it matters not whether
we uho the sines or tangents). But
we have also, by trigonometry,
BC : BA: : sine of BAG: sine of
ACB, or CBP, which last is the
apparent displacement caused by aberration. Thus it
appears that tl»e sine of the aberration, or (since the angle
is extremely small) the aberration itself, is proportional to
the sine of the angle made by the earth's motion in space
with the visual ray, and is, therefore, a maximum when the
line of si]iht is perpendicular to the direction of the earth's
motion."
(333.) " The uraiiogniphical otl'ect of ai>erration, then is
to distort the aspect of the heavens, causing all the stars to
crowd, as it were, directly towards that point in the heavens
which is the vanishing jioint of all lines parallel to that in
TIIEOUV or ABERRATION.
" which tlic earth is for tlio momont moving. As the earth
jiiovos round the sun in the ph»no of the ecliptic, this j)oint
must lie in that phme, 90° in advance of the earth's longi-
tude, or 90** behind the sun's, and shifts, of course, contin-
ually, describing the circumference of the ecliptic in a year.
It in easy to demonstrate that the oft'ect on each particular
star will be to make it apparently describe n small ellipse
in the heavens, having for its centre the point in which the
star would bo seen if the earth were at rest."
(3;{4.) " Aberration, then, aflbcts the apparent right as-
censions and declinations of al! the stars, and that by quan-
tities easily calculable. The formulae most convenient for
that purpose, and which, sj-slematicall}' embracing at the
same time the corrections for precession and nutation,
enable the observer, with the utmost readiness, to disen-
cumber his observations of right ascension and declination
of their influence, have been constructed by Professor
Hcssel, and tabulated in the a))])cndix to the first volume ol"
the Transactions of the Astronomical Society, where they will
be found accompanied with an extensive catalogue of the
places, for 1830, of the princij)al fixed stars, one of the most
useful and best arranged works of the kind which has ever
appeared." ^
(335.) " When the bod}' from which the visual ray ema-
nates is itself in motion, an ett'ect arises which is not, ])ro-
jterly speaking, aberration, though ic is usually treated under
that head in astronomical books, and indeed confounded
with it, to the production of some confusion in the mind of
the student. The effect in question (which is independent
of any theoretical views respecting the nature of light) may
be exi)lained as f(;llows. The ray by which wo see any
object is not that which it emits at the moment we look at
it, but that which it did emit some time before, viz., the
time occupied by light in traversing the interval which
weparatcs it from us. The aberration of such a bo ly then
THEORY OP ABERRATION.
67
" arising fromtho earth's velocity must l)o applied as a cor-
rection, not to the lino joining the earth's place at the
moment of observation witli that occupied by the body at
the same rroment, but at thut antecedent instant when the
ray quittel it. Hence it is easy to derive the rule given
by an*' .omical writers for the case of a moving object.
Fiom ihc hnoun hues of its motion and the earth's calculate
Its apparent or relative anyu/ar motion In the time taken by
Uyhtto traverse its distance from the earth. This is the total
amount of its apparent misplacement. Its ert'cet is to dis-
place the body observed in a direction contrary to its apj)a-
rent motion in the heavoris. And it is a compound of
aggrogaie etlect coiiHisting of two parts, one of which is the
aborration, projicrly so called, resulting from the compo-
sition of the earth's motion with that of light; the other
being what is not inaptly termed the equation of Hylit,
being the allowance to be made for the time occupied by
th« light in traversing a variable Hjjaco."
The hist section brings in a division of the subject not
immediately under consideration, but it is given here to
complete the explanation byHerschel, and also as belong-
ing to the general theory of (the so-cuUed) aberration
of light.
The explanation and illustration by Lardner are
included in those of llerschel; it will, therefore, suflice
to take the latter here for preliminary consideration.
The defmition of the meaning is by inference from ana-
logy, and the first illustration is that of the shower of
rain. The simple statement of fact, herein made, appeals
to the experience of every individual, and, as it is not at
onc«; contradicted by that experience, it may be termed
plausible; but upon more careful consideration, it will
appear, in respect to the application to be made and the
m
THEORT OF ABERRATION.
;i
inference intended to be drawn from it, that the statement
is not supported by fact. It is true that if a person runs
rapidly in a shower of rain, a drop of the water may come
in contact with his face, which would not have done so had
he stood still ; but it is suiely evident that the angle at
which the drop of water descended (or tlie angle at
which it rains) cannot have been altered by the person's
running, and this is the question at issue. A drop of rain
win occupy a certain time in descending through a spuco
equal to the distance from the upper part of a man's
forehead to his chin ; and if, during the time of that de-
scent, a man running brings his face in contact with the
drop, the effect is of the same kind as if the drop had been
suspended at that height from the ground at which it
comes in contact with his face. The additional sup-
position of the wind increasing the effect is, in regard
to the rain only, not open to the same objection, because
therein would be an actual cause operating to alter
(increase or decrease) the angularity of the rain's descent ;
the effect of the wind's force would combine with that of
the force of gravitation, and result in a compound effect ;
but in regard to the analogy, the supposition is entirely
false and inapplicable, because there are no grounds
for supposing that wind can divert or affect a ruy of
light ; on the contrary, it is quite established that the
fact is the reverse : the most violent hurricane dues not
cause a ray of light to deviate in tlie slightest degree
from its direction o'" ?.\>:{\r of incidence.
The illustratiun ( f th<* incMni .< tube, as shown, is not
altogether incorrect, u ic &h an »tn lioty it is very imperfect
and objectionable ; iu ! a i r.n explanation, very likely to
mislead the student. Takiiig the same figure, we will
THSORT OF ABERRATION.
s»
apply it, in the first instance, as follows (supposing the
inclined tube to be left out of the figure) : £SF represents
a plane moving horizontally with a certain velocity in the
direction EF. At P, in the perpendicular line APS, is a
ball falling vertically from A, towards S ; the propor-
Fig. 6.
Oa
o..
T
a
-Oc
-*««
tional velocity of the moving plane EF to that of the
falling ball is such, that a place on the plane will move
from Q to S, in the same time that the ball falls, from P to
S ; consequently, the boll P will fall upon the place Q. At
the same time the place S will have moved towards F,
and when P (the ball) arrives at S (or Q), S will have
arrived at T ; ST being equal to QS. The interposition
of the tube, in fact, alters nothing ; but it apparently com-
plicates the otherwise simple case — which is, that the ball
falls vertically and strikes the plane at right angles to its
position and motion : just the same as if the plane had
remained at rest, and the ball had been allowed to fall
from a place at the same height vertically over Q.
The analogy of the falling ball to light emitted from a
luminous body is very imperfect, because, whereas the
ball can only fall vertically or in some one angular direc-
tion, the rays of light from the luminous body are emit-
i.
60
TIIEORr OF ABERRATION.
ted, in every angular direction, in radiant lines from the
body as a centre. The conditions of the case are, there-
fore, essentially different from tliose of a ball falling ver-
tically ; whatever distance is assumed for A (supposing
it a star), rays of light from it will be continuuUy arriv-
ing at the earth in angular directions, depciident upon
its situation relatively to tlie place upon wliich tiie ray
h incident, and whenever the earth, moving in tlie
direction EF, arrives at Q, it will evidently nuu!t
rav of light which have just arrived from the star.*
By the illustration, the ball falls vertically upon the
moving plane ; now, su[>|»osing the ball is made to
descend at a detinite angle, as, for instance, through
the tube PQ, it would strike or conw in contact witli
tlie plane at its angle of descent {i.e. PQS), not, however,
at the place Q ; for, supposing the plane to be in nmtion,
and Q to have been at the base of the tube when the ball
commenced to descend, Q during the descent will
have moved to S, and another place on the moving
plane will receive the ball ; but this does not alter the
angle of incidence of the ball or of the light.
The correctness of this theory (aberration of liglit) may
be tested by the illustration of the method (>f determining
the Sim's parallax (as given in IlcrscloTs Astronomy, Fig.
art. 3oo). We will suppose the earth to be moving in
its orbit in the direction of the arrows; the effect of the
aberration of light (if real) would be, as explained in the
preceding (juotations, to shift the apparent place of the
sun from S to (some place) T. Conseipiently, if the
• We nre hero adopting, for the nionicu*, the Inngimgc of the theory,
in order to meet the argument on its own ground : in u strictly scicntilic
sense the ex{>ression is objectioniible.
-,-ji .*sicioi!i .
HELIOCENTRIC PARALLAX.
dl
zeniths of the places of observation were determined
independently of the sun's apparent place, the effect
would be to give a different parallax for the two places ;
that of BTC being greater than ATC ; but the zeniths
of the two places of observation must be determined
independently of the observed place of the sun, for
otherwise there could be no parallax ; the effect must
be therefore to increase the actual parallax — i.e. the
total apparent displacement — by the distortion due to
F!g. 7.
aberration. Now the parallax obtained by this geocen-
tric method is 8" 6 ; and the supposed displacement attri-
buted to aberration is 2()".5.
If now, leaving the case for particular examination in
the next cha[)ter, we discard for the moment the sup-
posed aberration of light as altogether imaginary ; and,
then, we assume those observed effects which have been
attributed by astronomers to aberration of light to be
really the effects of parallax, can we thus (from the total
amount of parallax) obtain an approximate meas-
r
62
HILIOOXNTRIO PARALLAX.
ure of the distance of the visible stars? The
quotation already given from Herschel's work shows
that the efforts of astronomers to obtain even such
approximate measurement .have been entirely un-
successful. These attempts were made by heliocentric
or annual parallax, in which the distance of the earth
from the sun serves for the base line of the triangle.
But this heliocentric parallax (as a trigonometrical pro-
cess) differs essentially from the geocentric ; nor is it
anywhere explained how the apparently great, if not
insurmountable, difficulty of thus directly obtaining
the parallax of a star, even if the distance was
less than the distance of the sun, has been over-
come. It is evident that knowledge as to the dis-
tance of a body is obtained in the geocentric method
by the two observations from places, at a definite and
known distance from each other on the earth, being made
at the same time ; but to obtain parallax by the heliocentric
method, it is impossible for two observers to be stationed
at different and distant places in the earth's orbit at the
same time, and therefore the method differs assentially
from the geocentric. It is true an observation can he
made from the earth at' one extremity, or at any place in
the orbit, and subsequently a second observation can be
made from the opposite extremity or from some other dis-
tant place in the orbit ; and the two observations may
be compared ; but does it follow, or is it to be expected
that the same result as by the geocentric method, or,
indeed, that any (reliable) result can be in this manner
obtained I If some of the stars moved with a known
velocity, and others were comparatively motionless, it
is not difficult to understand that observations of them
HBLIOCBNTRIO PARALLAX.
would have a diffiirentiaJ value from which further know-
ledge might be obtained. But as all the stars are rela-
tively (iilnioat> motionless, it doo!« not immediately ap-
pear wherv the standard of cumpurison is to be found, or
whoHtM* th« diflTeivinlHl iinsflo to Uv obtained wpon which
to btt«r \\w ('(Mnpntiition. Theiippiirent motion of all the
gta»"»* V HVjpposing tho distanct' tliem aM to be very great)
|i necessarily nearly tlie same. An ennentially distinct
basis .'or the (^unputatiou has, therefore, to be sought,
and may be found in observing the relative positions of
the sun and the stiir to that of the earth when the eartb
is at some di'tinito place in its orbit, as for example,
the central ploc» eqnidly distant from twc definite extre-
mities of the orbit; and then when the earth has arnved
at a distant part of the orbit, observing tiie alteration
in the relative angidar position of the earth and sun,
and the earth and star, respectively.
In this manner the h#'lio-centric parallax of the star
may be obtained ; and, a? we will presently show, by a
modification of the same method two definite compara-
tive angles may be obtained proport'onal to each other in
the same ratio as the distance of the star from the sun is
to the distance of the earth from the sun. Since the last
is a known quantity, the distance of the star may be thus
measured.
The collective parallactic result of the earth's pro-
gressive change of position throughout a complete revo-
lution known by the tenn annual parallax is thus
described by Dr. Lardner: —
Lardner's Handbook of Astronomy :
(2442.) " Annual parallax. — If the earth he admitted to
movo annually round the sun, as a stationary centre in a
u
HELIOCENTRIC PARALLAX.
circle whoso diamotor muHt liavo the vast magnitude of 209
millions of milcH, nil obHorvera placed upon the earth, seeing
distant objects from points of view so extremely distant one
from the other as nrr opposite extremities of the same
diameter of such a circle, must necessarily, as might bo
supposed, see those objects in very ditforent directions.
" To comprehend the effect which -
might be expected to be produced
upon the apparent place of a distant
object by such a motion, lot E E E E
(fig. 718) represent the earth's annual
course round the sun as scon in per-
spective, and let bo any distant ob-
ject visible from the oarlh. The ex-
tremity E of the line EO, which is the
visual direction of the object, being
carried with the earth round the circle
E E E E \\\\\ annually describe a cone
of which the base is the path of tho
earth, and the vertex is tho place of
the object 0. While the earth moves
round the c\vc\o E E, tho lino of visual
direction would, therefore, have a cor-
responding motion, and tho apparent
place of tho object would be succes-
sively changed with the change of di-
rection of this line. If the object bo i\
imagined to be projected by the 03-0
upon tho firmament, it would trace
upon it a path 0' 0" o"\ which would Fig. L. 718.
bo circular or elliptical, according to the direction of
the object. When the eailh is at Ey the object would
be seen at 0; and when tho earth is at J?, it would
be seen at 0". The extent of this ai)parent displacement of
HBLIOCINTRIC PARALLAX.
6&
m
I bo
" tho object would be mooHurud by the angle E £, which the
(liamotor E Js! of the eorth'H path or orbit would subtuml at
tho object 0. It haH boon Htuted that, in general, tho appa-
rent dirtplacenient of a distant viBiblo object, produced by
any change in tho utation from which it \n viewud, ia cnllod
parallax. That which in produced by tho change of position
duo to the diurnal motion of the earth being called diurnal
parallax, the corres]ionding diaplacement duo to the annual
motion of tho earth in called annual parallax."
The general conclusion come to is the same as that
expressed in the r|uotation previously given from Iler-
schel's work : namely, that no parallax of any of the stars
has be';n obtained in this way. On careful examination,
however, it will appear that all the parallax observations
in recent times have been rnude with a foregone conclu-
sion that no parallax wos attainable ; or that, if any was
attainable, it must necessarily be an extremely small
amount, not exceeding, at the utmost, the sine of 1".
The consequence seems to have been that any quantity
of parallax obtained exceeding this 1" has been set down
to aberration of light, or to error.
The Encyclopedia Britannica — art. Astronomy:
" Suppose, for oxam])le, wo observe a star situated in the
plane of the odiptic. When tho earth is at that point of
its orbit, between tho sun and the star, where the tangent
to the orbit is perpendicular to the visual ray (which, on
account that tho star has no sensible parallax, alwaj's main-
tains a paralhtl direction), the apparent place of tho star
will be 20".4 to the westward of the true place ; so that it
will appear to have an oscillatory motion on tho ecliptic,
the rang© of which is 40".8, and the period exactly a year.
Half way between these two points, the tangent of the orbit
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66
HELIOOENTRIO PARALLAX.
" is parallel to the direction of the ray of light, and conse*
quently there is no aberration. When the star is not
situated in the ecliptic, it will suffer a displacement in
latitude as well as in longitude. To render this more intel-
ligible, let E E B (fig. 28) be the ecliptic ; E, the earth ;
and A, the true place of a star situated at any altitude
above the ecliptic. In the direction EA, take Ea to
represent the velocity of light; a b, that of the earth,
and in a parallel direction, that is, parallel to the tan-
gent to the ecliptic at E ; the line Eb will now be the
apparent visual ray, and the star
will seem to be situated at B. Sup-
pose the earth to be placed at diffe-
rent points of its orbit; the lines ^a
will be all parallel to each other, on
account of the infinite distance of
the star A ; the lines a b will vary
little in magnitude, because they are
very small in comparison to Ea,
but their di^-ections will undergo
every possible change being paral-
lel to the tangent at E. At the two
points of the orbit where the tangent is parallel to EA, the
two lines Ea and a b coincide, and consequently there is no
aberration. Let us noxt suppose the star to be situated in
the pole of the ecliptic. In this case the visual ray is con-
stantly perpendicular to the direction of the earth's motion,
so that the star will always appear at a distance of 20".4
from its true place, or appear to describe a small circle about
the pole of the ecliptic. In all other situations, out of the
ecliptic, the star's apparent path will be an ellipse, the
major axis of which, parallel to the plane of the ecliptic
is always 40".8, while the minor axis varies as the sine
of the latitude." ,,.
I^g. E. B.
ABERRATION AND PARALLAX.
67
Jjardner's Handbook of Astronomy, page 179 :
(2447) " Close resemblance of the effects of Parallax to
Aberration. — Now, it will be apparent that sUch phenomena
bear a very close resemblance to those of aberration already
describ'^d. In both the stars appear to move annually in
small circles when situate 90° from the ecliptic; in both
they appear to move in small ellipses between that position
and the ecliptic ; in both the eccentricities of the ellipses
increase in approaching the ecliptic ; and in both the ellipses
flatten into their transverse axis when the object is actually
in the ecliptic."
(2448. "Yet, aberration cannot arise from parallax. —
Notwithstanding this close con'espondence, the phenomena
of aberration are utterly incompatible with the effects of
annual parallax. The apparent displacement produced by
aberration is always in the dii-ection of the earth's motion ;
that is to say, in the direction of the tangent to the earth's
orbit at the point where the earth happens to be placed.
The apparent displacement due to parallax would, on the
contrary, be in the direction of the line joining the earth
and sun. The apparent axis of the ellipse or diameter of
the circle of aberration is exactly the same, that is 20".42,*
for all the stars ; while the apparent axis of the ellipse or
I
I
• In the preceding quotation from the Encyc. Britann., this quantity is
correctly stated as 40".8; the writer of the art. therein realizes that the
supposed effect would, in the case of an equatorial star, manifest itself not
as an ellipse, but as a linear displacement ; the star appearing to have a
reciprocating motion backwards and forwards through 40 .8. Surely the
fnndaniental fallacy in the theory becomes here very evident, namely,
that such an optical effect cannot take place unless there be an angular
alteration in the relative position of the star and the observer ; whereas, if
there be any such alteration in tbeir relative positions, the effect is due to
parallax and not to aberration,
MMi
68
HELIOOENTBTC PARALLAX.
diameter of the circle, due to annual parallax, would be dif^
forent for stars at different distances, and would vaiy, in
fact, in the inverse ratio of the distance of the star, and
could not, therefore, be the same for all stars whatever,
except on the supposition that all stars are at the same dis-
tance from the solar system, a sujiposition that cannot be-
entertained." ;
But the illustration L, Fig. 718 of page 64, taken
from Lardner's own work, will show that the observed
effect of aberration, as described in the Encyclopedia,
amounting to 20".4, is precisely of that kind which
would be occasioned by parallax.
I i
(8) A Direct Method of obtaining Parallax of the Dis-
tant Stars.
Let us now, resuming the assumption, tentatively, that
the theory of the aberration of light is not based on
fact and consequently untenable, consider the amount
20".4, attributed to aberration, as an approximation to
the average or mean parallax of a great number of the
visible stars; and, with this suppositious quantity as
datum, compute, by the direct method of heliocentric
parallax, the average distance of those stars from the
sun. The modification of the method which appears to
us the most simple and advantageous is illustrated in
fig. 9, Supposing the star to be in the equatorial plane
of the sun at a distance Crom the earth equal to the
diameter of the earth's orbit, the lines of junction would
evidently form an angle of 46 degrees (the earth having
moved from one extremity of the orbit to the other).
DISTANCE OF THJi! STARS.
69
We obtain, therefrom, the proportion : —
As 45° : 20".4 : : distance of the star: 190 million
miles. Consequently 90,529,518 million miles, or about
953,000 times the distance of the sun from the earth, is
the quantity which, subject to the supposition, thus
represents the average or mean distance of the stars.
'•»
'
F[G. 9.
• 1
i
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\
- (
/
)
I
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■'■■ ■ ^ ^ \
--^-^
•'. '
Note. — If tho star bo vertically above the earth, the same
method jWill apply ; but in that case, for correct application
thereof, the successive observations of the relative angular
position of the star must be made only at each of the nodes...
that is, when the earth, in its ascent and descent, is passing
and repassing through tho equatorial plane of the sun.
1
fl
I
E;h
CHAPTER III.
THE ABERRATION-OF-LIGHT THEORY.
We have already in the preceding chapter, page 50 to 57^
put before the reader the particulars of the theory of aber-
ration as stated and explained in the works of Dr. Lar Jner
and Sir John Herschel. We have also in the same chap-
ter stated certain preliminary objections to the theory.
It is, in the fir»t place, desirable to show, as far as can
be done, that the theory (of aberration) on its own ground
is irreconcileable with the observed facts of astronomy.
But, now, the very circumstance which should, as we
think, have prevented the adoption of this theory,
constitutes a difficulty in the way of demonstrating its
unreality.
(9.) The Conditions and Requisitions of the Theory.
That circumstance is the absence of any actual fact or
evidence upon which the theory is even presumably
based. The theory is wholly suppositious. It is an infe-'
rence from a supposed analogy ; and is applied to casea
which in the great majority of instances cannot be
brought to the direct test of fact. Some instances, how-
ever, where such a practical test can be applied, we shall
presently bring under consideration. Before doing so,
in order that the reader may have the conditions of the
theory distinctly in mind we will briefly take again the
exposition and illustration of the theory, already quoted
in Chapter II., (page 39), from HerscheVs Outlines of
Astronomy ; ,
" This cause is what is called the aberration of light ;
a singular and surprising effect arising from this, that we
ABBRRATION-OP-LIGHT THEORY.
ft
" occupy a station not at rest, but in rapid motion, and
that the apparent directions of the rays of light are not
the same to a spectator in motion as to one at rest."
" Suppose a shower of rain to fall perpendicularly in a
dead calm ; a person exposed to the shower, who should
stand quite still and upright, would receive the drops
un his hat which would thus shelter him; but if he ran
forward in any direction, they would strike him in the
face. The effect would be the same if he remained still,
and a wind should arise of the same velocity, and drift
them against him."
Suppose a ball let fall from a point A, above a hori-
zon Lai line E F, and at B, were placed to receive it the
open mouth of an inclined hollow tube P Q ; if the tube
were held immoveable, the ball would strike on its lower
side J but if the tube were carried forward in the direc-
tion E F, with a velocity properly adj usted at every instant
to that of the ball, while preserving its inclination to the
horizon, so that when the ball in its natural descent
reached C, the tube should have been carried into the
position R S, it is evident^ that the ball would through-
Biill
I!
i1
12
ABERRATION-OF-LIOIIT THEORY.
out its whole descent be found in the axis of the tube ;
and a spectator referring to the tube the motion of the
ball, and carried along with the former unconscious of its
motion, would fancy that the ball had been moving in
tlie inclined direction R S of the tube's axis."
(10.) Practical Application of the Theory.
Let ns now, taking this explanation, test it by appli-
cation to the actual phenomena of the stellar universe.
Fig. 11 shows the sun occupying the central place, the
earth's orbit and four stars r»med respectively Alpha,
Beta, Gamma, and Delta, which belong to the distant
constellations and are in the plane of the sun's equator;
and which are so situated, relatively to each other, that
a line passing through the centre of the sun and joining
<?ither two of those opposite to each other, is at right
angles to a line passing also through the centre of the sun
and joining the other two. The actual places of the four
stars are denoted by the letters a, a', a", a'". We will
suppose'the earth to be at m, at the eastern extremity
of its orbit, and to proceed in its orbital path around the
sun. An astronomer, on or near the earth's equator (the
earth being at the place m in its orbit,) observes
each of the four stars and compares their apparent places
relatively to each oiiher.* Now their actual places are,
as already stated, those denoted by the letters a, a', a",
a'"', but where will their ap^^^rew^ places be, according to
the theory of aberration as explained in t>e foregoing
quotftion from Herschel's Outlines ? The observer first
looiis towards Alpha. Since he is moving directly
* Evidently he could not observe these stars all \t the same time of the
day, although he could do so at dififbrent times of the same day. It is
meant that he determine the longitude of each star successively, when the
earth is at or near the place indicated. V
ABERBATION-OP-LiaHT THEORY.
73
towards that star, there is, by the theory, no aberration,
and he sees it in its actual place at a. He next observes
lieta. . . .Since the earth's motion (of about 19 miles a
second) is at right angles to a line joining that star and
the sun, tlie aberration is, according to the theory, here
effective to the fullest extent; and, instead of seeing Beta
at a', he sees it at c'. He next turns to Cramma. . . .And
since he is moving directly away frpm that star, he sees
it in its true place at a". Lastly, he looks at the star
Delta,. . . .here again aberration is fully effective, and he
sees Delta not at a'", but at h'". We now suppose the
earth to. have proceeded to the northern extremity of its
orbit, at n ; and the astronomer repeats his observations
of the four stars successively. By the theory of aberra-
tion (Ilerschel's foregoing explanation) he will find that
each one of the four stars has moved to another place ;
for he is now passing Alpha at right angles to a line
joining that star and the sun, and therefore, by the theo-
ry, he sees Alpha at h. And he is moving directly
towards Beta, which he now sees in its true place at a'
. Similarly Gamma has moved its apparent place from
a", to c", and Delta has retired to its true place at a'".
The earth proceeds, and arrives at the western extremity
of its orbit at o. The astronomer renews his observa-
tions. . . .The aberration theory tells us where he will
find each of the four stars — namely, Alpha will have gone
back to a. Beta, which was at the time of the first ob-
servation at c' and at the second observation had moved
to a', will now be found at 6' . . . And similarly of the
others. Gamma will be found at a" and Delta which was
first at V" and then at a'" is now at c"'. Lastly, the earth
having arrived at the southern extremity of the orbit, the
74
ABBRBATION-OF-LIOHT THIORT.
observer finds that each star has again shifted its place
. . . Alpha must now be seen at c ; Beta at 'a ; Gamma
has gone to b" ; and Delta back again to a'".
Here then is the statement of a case as to which the
practical astronomer may be called upon for* evidence.
How stand the facts of observation ? for this is simply a
question as to the places at which the observer sees the
stars. Do the facts of observation support the aberra-
tion-theory ? Do the constellations in or near to the
solar equatorial plane thus continually, independently
of any parallax (which is quite distinct from the ques-
tion here at issue,) shift their positions backwards and
forwards relatively to each other, as the earth progresses
in its orbit ; at one time approaching nearer to each other,
and again receding from each other ; each of them shift-
ing its place throughout an arc • of 40". Let the practi-
cal astronomer give evidence in the case. Take four
constellations situated, relatively to each other, as we
have supposed ; such, for example, as Gemini, Pisces,
Sagittarius, and Virgo. Any one star in each of these
four constellations may be chosen to try the case. Can-
not the question be decided by positive evidence,
whether the imperative requisition of the aberration-
theory (as stated by Herschel) is in this particular case
fulfilled or not T
There is a corollary to the aberration-theory, or, at
least, what, .s it seems to us, must be a corollary if the
theory has any substantiality or definite consistency in
itself, which, if stated distinctly and directly, can scarcely
fail to startle the practical optician. It is. . .that the angles
* The angle of aberration is 20" 5, which has to be doubled
because it is at first fully effective in the one direction and
afterwards in the reverse.
K. N.-
■:i
i1^
\
/
/
/
m
\
■^
C .<#
,11
ABBRRATION-OP-LIOHT THIORT.
fl
of incidence and reflection are not always equal. For the
angle of vision, or the angle under which the eye perceives
the luminiferous body, cannot be otherwise than equal to
the angle of reflection, and this angle of vision is not,
according to the aberration-theory, the true angle of
incidence if the station of the observer have a progressive
motion in such a direction as to form witH the direction
of the luminiferous body an angle not much less or greater
than a right angle. Therefore the angle of reflection
from the earth of the solar-light must always difler from
the true angle of incidence f
(11) The Nature of Light as assumed by the Tlieory.
But the statement of this corollary brings again under
notice the anomalous character and, indeed, contradic-
tory nature of the qualities which are attributed to
* Light ' accordingly as the exigencies of the one oi* the
other of the three theories — the velocity, the aberration,
and the undulatory, theories — require a decidedly ma-
terial nature subject to the laws of matter, or a semi-
material nature subject to some of those laws and exempt
from others, to be assigned to tho atoms or waves of
' light.* • It is, perhaps, in the exposition of the aber-
ration-theory that the want of a definite and real founda-
tion becomes most immediately manifest. Referring
again to Sir John Herschel's explanation, we have the
* The meaning of the ezpreision ' Light ' has now become, in some
measure, involved in the vague and indefinite meanings which belong ia
common to these theories. It sometimes means luminous particles or atoms
of luminous matter travelling with incredible Telocity.. ..sometimes means
the particles of a purely suppositious fluid, called Ether, vibrating with
iuoonceivable rapidity... sumetimes means an effect produced by impact oC
the luminous matter on the eye " , > /;;;;;;
11
1
II
M
n;S
■"!m
1 II 1 '1
I mm
I'm
1
1
76
MATERIAL CHARACTER OF THE THEORY.
analogy of light to the shower of rain and to the falling
ball, that is.. to matter in the liquid and in the solid
condition. Something called ' light ' has motion and velo-
city, just as the earth moves iii its orbit v/ith a definite
velocity. . . .it enters the advanced end of the telescope
tube, which is moving rapidly forwards, it is in the tube,
moves through the tube, and, emerging from the posterior
end, comes in contact with the eye of an observer, or
with tlie ground ; or, again, the observer, who is moving
forward with the earth upon which he is stationed, strikes
his eye against the descending ray of light, just as the per-
son whorunnitig forward strikes his face against the falling
rain, .there is impact ; the person whoue eye receives the
shock, which is a compounded result of the motion of the
ray and ol his own motion, infers the position of the lumi-
niferous body from the compounded direction in which
that shock is received, and hence is in a measure deceived
or misled. The supposed analogj' is here to another
form or hind of matter in motion. The fundamental theory
of light to which it belongs, appears to be the corpuscular
theory of Sir Isaac Newton, in which extremely small
particles of luminous matter are ejected from the lumi-
niferoiis body and move in right lines with enormous
velocity, rather than the undulatory theory. At last we
find men the most experienced, eminent and distinguished
in the sciences of astronomy and optics, as Sir John
Herschel, in order to reconcile these theories of light
with the actual and known facts of the phenomena, en-
deavouring to suppose the existence of a description of
matter from which all the properties and qualities of mat-
ter are absent. The note to the ' Outlines of Astronomy,'
in which Herschel adopts and endorses tie doctrine of M.
Doppler on this subjecct, and which we have already
IM3IATERIAL MATTER I
77
lid
|0-
te
ve
lor
or
tm
;es
3r-
quoted (page 54), willserve to exemplify this strange hy-
pothesis of an immaterial description of matter. " This
condition is indispensable. Without it we fall into all
those difficulties which M. Doppler has so well pointed
out in his paper on Aberration. If light itself, or the
luminiferous ether, be corporeal, the condition insisted on
amounts to a formal surrender of the dogma, either of the
extension or the impenetrability of matter ; at least in the
sense in which these terms have been hitherto used by
metaphysicians. At the point to which science is
probably arrived, few will be found disposed to mention
either the one or the other."
This supposition of matter without any of the pro-
perties of matter seems to be precisely equivalent to
supposing an animal without head, body, limbs, bones,
flesh, or, in short, without any of those things which
especially pertain to an animal. It is true there is, in
the foregoiiig, a sort of saving proviso by Herschel, ' if
liffht be corporeal,' — but then, if it be not corporeal,
what becomes of the undulatory theory, what of the
theory of aberration, and what of the velocity-theory of
light ? all of which are upheld by Herschel.
(12) Aberration a Dynamical Theory.
Let us return again to the astronomical theory ; and
submit to the astronomer the following case: We will
suppose that the areal (absolute) velocities of the three
planets, Venus, Earth, and Mars, are exactly equal, and
that their angular velocities relatively to the Sun are so
proportioned (by their icspt^tive distances from the
Sun) that whilst the Earth is moving through an arc of
six degrees, Venus moves through ten degrees, and Mars
through only four. Now, taking the maximum angle of
IF.
78
ABERRATION A DTNAMICAL THEORY.
I
:i.|
aberration at 20" for the terrestrial observer, what will
be the angle of aberration for the inhabitant of Venus
and of Mars respectively ? From Herschel's illustration of
the inclined tube and falling ball, the angular velocity
is that which must determine the angle of aberration ;
for, if the distance of the plane E. F., upon which
the ball falls, from the starting point A. be doubled,
and the plane, move with twice the linear velocity
relatively to the point A., the same inclination from
the perpendicular would still be given to the tube.
To appreciate the wlaole case, it must be remembered
that this illustration of the tube and ball illustrates
the artificial idea upon whtch the aberration-theory is
based only, and does not illustrate the case of an eye or
of a telescope directed towards a body outside the Earth :
if, for instance, a stationarj'' object be supposed a few
miles from the Earth which is moving, it is evident that
the angular position of a telescope constantly directed
towards the object would require to be constantly chang-
ed. Now in Herschel's illustration of tube and moving
plane, the same inclination of the tube is preserved
throughout the movement ; therefore the illustration is
defective, and it is also deceptive, because the theory of
aberration itself and Herschel's own exposition of it,
each expressly supposes {see Fig. 13) such an appre-
ciable alt ration in the relative position of the recipient
body wl ich moves, and the luminiferous body which
remains stationary. The tube or telescope is always sup-
posed to advance relatively to a perpendicular drawn
vertically or horizontally, as the case may be, from the
object to the plane beneath it ; for the effect claimed, as
stated by Herschel, is dependent upon and arises out of
this motion of the plane relatively to the object at rest.
1
!
^■j i
> i
f
■*3
1^
A-
-^---"^
I '
ABERRATION A DTNAMIOAL THEORY. ^
Now, if the consideration of the case in this manner be
pursued with attention, it will become evident that the
theory of aberration breaks down altogether / because if
the body move relatively to the perpendicular drawn from
the object to the plane, the angular position of the tele-
scope must be changed, for if it be not changed the suc-
cessive rays of light cannot enter the tube ; and, if the
body has no such relative movement, there can be
no effect such as claimed, .but, if the angular position
of the telescope requires to be changed, whether it be to
increase or decrease the inclination* of the telescope, any
optical effect necessitating such a change must belong to
parallax. The especial point to which the attention
should be directed, is that, if there be no appreciable
alteration in the relative po&itions of the luminiferous
body and the eye, so that the telescope having the same
inclination, constantly receives light from the object at
the same angle throughout the movement of the plane,,
there can be no appreciable aberration even if the possi-
bility of the theory be admitted in other respects. And, if
there be an appreciable alteration in the relative position,
the effect must be parallax and in the reverse direction to
that claimed for aberration. When the whole case is cor-
rectly apprehended, the utterly unreasonable character
of the general result supposed becomes apparent j for, ii a
movement of the earth through only nineteen miles
of orbit produces aberration, some proportional altera-
tion in the relative angular position of the object and the
■M
* As the telescope approaches or passes the perpendicular drawn from
the object to the moving plane upon which the observer is stationed, paral-
lax will require the inclination of the tube to be altered in the opposite
direction to that which aberration would require. ,
80
THEORT AND FACT.
! i
W •
ii :ii
eye must manifestly take place ; and if the one effect be
appreciable so must the other effect be also appreciable ;
but what, if instead of the plane moving nineteen miles,
it move nineteen hundred or nineteen thousand, nay,
nineteen million and even, a distance several times greater
than nineteen million miles, and yet without any appre-
ciable alteration in the relative angular position of the
eye and the object ? A claim of 1" aberration for every
1" angular alteration of position (parallax) might appear
to be primarily reasonable until shown to be otherwise,
but a claim of 20" aberration and no appreciable altera-
tion in the relative angular position is manifestly inad-
missible. Even Herschel's own illustration evidences
negatively that there cannot be any such effect ; hence. .
Note (a.) The theory of aberration is a dynamical the-
ory in which the very meaning of the term motion, as a
relative expression, seems to be imperfectly appreciated or
misapprehended. One body moves relatively to another.
A body moves with a certain velocity relatively to '>. stan-
dard of velocity. There is angular and linear velocity, and
each of these is relative. It may be shown that if tlie
earth relatively to the distant stars has no appreciable
motion and no velocity such as contemplated by the
theory, not even a suppositious case of aberration has
been made out in respect to those stars.
(13) Distrust in the Gift of Sighc required by the
Aberration Theory.
Before leaving the theory of aberration — let us, refer-
ring to either of the illustrations we have given of the
eclipses and occupations of Jupiter's satellites, once more
briefly note what the student of astronomy is impera-
tively required by that theory to understand in respect
THEORY AND FACT.
81
be
lies,
fay,
iter
)re-
Ithe
tevy
fear
(ise,
3ra-
tad-
tces
to those phenomena.* It is. .that he is not to suppose
what he appears to see is actually taking place when and
as he sees it, but that it is merely certain reflections of
light, and interruptions and interferences with light,
which have been occasioned by something which has
happened about 35 minutes or 50 minutes previously,
according to the place of the earth in its orbit at the
moment of observation : for example, if he appears to
see the satellite just entering the shadow, he is to believe
that the satellite has really entered the shadow some
forty or fifty minutes earlier, and, if it be the satellite
nearest the planet, is already almost at the middle of the
eclipse (or occultation.) But the shadowy messengers
of light, belonging to the aberration-theory, have occu-
pied all that time in bringing him the intelligence of
what formerly happened ; indeed, however, he has not
yet sufficiently distrusted his eye-sight — this is pnly a
general distortion and displacement of everything, which
belongs of right to the velocity-of-light theory, aberra-
tion proper has not yet come into play, it has its functions
to perform, and, seizing the shadowy record of the past
event just as it reaches his eye, distorts it afresh by the
angle of aberration proper, making it appear that the
event, of which intelligence has at length arrived, hap-
pened at some place other than that at whi ^h it actually
occurred.
We think the student, who has apprehended that this
is the demand made upon his faith by these theories,
which say to him. .'put your confidence in us, distrust
your eye-sight and beware lest it deceive you,' and who,
• This refers to our ParC Fifth which has for its subject the undulatory
and velocity theories of light.
i
:82
EXPERIMENTAL INVESTIGATION.
i i
1
I :
1 I
then taking his telescope, reads apparently, not the
record of the past event, but the event itself actually
occurring as he watches the clear definition of each
successive phase, will act reasonably if he listen atten-
tively to the counter-claim made from within ' to put
confidence in his eye-sight, to distrust the theories and
beware lest they deceive him-'
Note (6.) — We suggest that an advantageous means
of practically testing the truth of the Aberration-theory
may be found in the observation of one of the lesser of
Jupiter's Satellites.
Let the Earth be supposed (preferably) at a pLce in
its orbit near to opposition ; let the (apparent) moment
of the satellite passing the centre of the planet during oc-
cultation be carefully determined from the ingress and
egress : then, let the moment of passing the centre of
the planet at the opposite extremity of the Satellite's
orbit, namely, the central point of the transit; and,
then, the central moment of the succeeding occultation
be determined.
If there be truth in the theory of aberration there
must necessarily be a distinct (apparent) difference be-
tween the two semi-revolutions ; for, at the occultation
both the Earth and the planet's satellite are moving in
the same direction and there will be virtually no aberra-
tion ; but at the transit, the Earth moving in one direc-
tion, the satellite moves in the reverse; and, consequent-
ly, the effect of aberration must be increased and should
considerably exceed the 20".5.
As the angle of aberration would be an addition to the
one side and a deduction from the other, the difference
between the two semi-revolutions would be more
than 1'.
RADIATION INTO SPACE.
83
Ithe
\lly
iCh
m-
mt
bd
ins
ny
of
Note (c). — On the question, .vjhethera luminiferoiis and
calcri/erous body, in the absence of a recipient (or recipro-
cating) body, radiates light and heat continuously into
space. . . .*
How has it been ascertained that the sun radiates light
into space, and in every direction alike ? Gravitation is
also an influence which is communicated from the sun to
the planet, or is intercommunicated between them ; and
it may also be said to be emitted by the sun. Is, then,
the sun supposed to emit or radiate gravitation into
space ? Or, is it only emitted in the direction in which
there is an aggregated mass of matter, to receive and
reciprocate that influence ? Tf the latter, then, suppos-
ing we dismiss all foregone conclusion and prejudice,
does it appear so certain that the influence which causes
light may not be in the same case ?"
We wish now, without introducing the case into our
main argument, to point out that both the theories,
of Aberration and Velocity of Light, are also depen-
dent upon the assumption of the continuous radiation
of light into space by tki sun or other luminiferous
body. We do not mean that the assumption affords any
evidence or basis to support the theories but the theories
require and are dependent upon the assumption. For,
if the assumption be not true in fact, it will follow that,
since, by each of the theories, the communication of light
requires time (i. e. light has velocity) a star of which the
distance from the earth exceeds a certain limited amount
must be invisible from the earth. The earth travels in
its orbit with a velocity (more than 1000 miles a minute)
which will in about 8 minutes remove its entire bulk
out of the space which it occupied at the commencement
* This question will come uader consideratiou in our Fart Fifth.
84
RADIATION INTO SPACE.
of that time. If, therefore, the luminiferous body be at
such distance from the earth, that light (being assumed
to have velocity) requires more than about 8 jninutes
to reach the earth, the body, during a great part of
the earth's orbital revolution, would be invisible, because
the rays if emitted towards the earth would be too late to
arrive and would be projected into space or vacancy. But
notwithstanding the enormous and incredible velocity
assigned to light by the theory, a quarter of an hour would
not nearly sutlice for light to reach the earth from tlie
very great majonty of those stars which are in fact visible.
In many cases the earth would have ample time, not only
to get out of the way of the luminous matter, but to
make one or mo"e complete revolutions in its orbit, and
might thus occasionally and accidentally (so to speal^)
return to its former place just wlien the rays were arriv-
ing.*
Now, is the assumption of continuous radiation into
space established on certainty ? Is it quite reliable, unas-
sailable, and not open to any doubt whatever! Or is it
itself an unproven theory, plausible, certainly, at a time
when the known facts belonging to the subjects of light
and radiant heat were comparatively few, but subject
now to grave objection and doubt I Such grave objec-
tion and doubt respecting the assumption we, for our-
selves, entertain. We remember the very reasonable ob-
jection taken by Sir D. Brewster to the undulatory theory
of light t (against the form of which objection as irrever-
• Sir John Herschel estimates the time, required by the light from
some of the most distant (visible) stars to reach the earth at about 2000
years.
t Part Fifth (of this series).
HELIOCENTRIC PARALLAX.
85
at
lied
of
|U8e
to
flit
Hty
|ul(}
cnt we felt called upon to protest) ; an objection of the
same character seems to us to apply with ecjual force to
the case now under consideration. It does not seem
reasonable, bearing in miiiu the properties and tpialities
of light and radiant heat, their great i'liportance, and tlic
grand and invaluable services rendered by them in the
economy of nature we say, it does not soeni reason-
al)le to suppose that a large proportion of tlie ligiit and
lieat radiated goes to waste. . .is radiated and lost; yet
such is the meaning of radiation into space. If there be
a recipient, it is not difficult to understand that there need
not be loss, the heat or light is received and (reciprocat-
ed) 'returned in the rime or in some other (mode) condi-
tion of force. But radiation into space or vacancy means
no return.*
There is besides, as noticed before, the kindred and
analogous force of gravitation. Do masses of aggregat-
ed matter gravitate into space? No then why
sliould it be positively concluded that they radiate into
space ?
(14) Direct Heliocentric methods of ohtaininy Parallax
of the distant Stars.
Referring to the illustration in the preceding chapter,
a correct method of computing the distance of the stars,
. . . .we are strongly of opinion that the method there
t We are mindful of Dr. Wells' theory of dew, but acceptance of this
philosophical and felicitous explanation of the phenomena does not neces-
sitate the supposition of rf.diation of heat into space from the surface of
the earth there are the stars in sufficient number to serve on a clear
night as recipients, although we incline to the opinion that the escape of
electricity, in some condition of force other than that of free caloric, into the
atmosphere, causes that reduction of temperature on the surface of the earth
which condenses the wat^^ry vapour.
II
86
METHOD OF niLlOrCNTRIC PARALLAX.
1]
indicuteil of ascertaining the parallax is not only prac-
ticable but is also tiie most simple and direct method.
Repeating the illustration of page 69, on the larger
scale of Fig. 12 (A.) . . The Earth may be supposed
at any definite place m. in its orbit, at which i)lace it i»
found by careful observation that a certain star in, or
not far from, the solar equatorial plane, is so situated
with respect to the sun that the vertical plane, joining
the centre of the earth and sun, is at right angles to the
vertical plane joining the centre of the earth and star»
From the time of that observation, the Earth having
made an orbital semi-revolution (exactly), the angle con-
tained by the vertical planes is again determined by care-
ful observation, and the difference between the two, i.e.
the difference between the last angle and a right angle, is
the parallax.
For ourselves, we are quite sure that aberration of
light is a mere phantom of the imagination, but even
those, who for the present are persuaded that human
sight is deceived in that manner, will allow that aberra-
tion could not interfere with parallax ascertained by the
method here proposed, for tbe earth would be at the
time of the one observation directly receding from, and
at the time of the other, directly approaching the star
(or vice versa) and, therefore, by the theory there would
be no aberration. Or, again, supposing the north polar
zenith of the earth, when passing through the sun's equa-
torial plane, be accurately determined, and at the com-
pletion of 8 semi-orbital revolution of the earth the same
place be found, the differenee from a right angle with the
sun's equatorial plane will be the parallax of the Pole-
star ; from which the approximate distance of the star
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PARALLAX OF THE STARS.
8r
would be readily obtainable by the simple computation
shown at page 69 See Fig. 12 (B.) In this
case also, 'aberration' could not interfere ; for by that
theory its effect would be to shift the star's place back-
wards in the vertical plane joining the star and the earth,
but it would not affect the angle formed by that plans
and the solar equatorial plane. Whereas the parallax
would be the deviation of the plane from a right angle
in consequence of the removal of the earth to the oppo-
site extremity of the orbit.*
(Note. — ^We have taken the riglit angle to render the illustration
nioreclear, but, if the angle differed from a right angle, the difference
between the first and second observation would still give the parallax ;
if, however, in the angle first observed the inclination be towards the
sun, the star might be a truly solar pole-star, and in that case no
parallax would be thus obtained because the angle would be the same
from the opposite sides of the earth's orbit.)t
Another method,, by which we opine the approximate
parallax of the stars may be obtained, is by the com-
parison with each other of stars situated 90 degrees
apart on or near to the celestial equator. Fig. 11 may
serve well to illustrate this method. Suppose the station
of the terrestrial observer to be at p. and let him note
the actual and relative localities of the stars Delta and.
Alpha. At the expiration of six months his station
having arrived at n. let him note again the actual and
relative localities of the same two stars. Parallax will
* Evidently the absolute right angle is not indispensable. Take Polaris,
and, determining the exact deviation from a right angle with the equa-
torial plane when the earth is at the one node, then determine the increase
or decrease in that deviation when the earth has arrived at the other node.
t We are in this example assuming that our demonstration of the per-
pendicular terrestrial axis of rotation, parallel to the solar axis, will neces-
sarily be admitted, but, even otherwise, the method admits of modification
accordingly.
«8
PARALLAX OF THE STARS.
have shifted the apparent place of Delta from h'" to c"
but the place of Alpha will have undergone no change,
for the observer, on both occasions, sees Alpha at its
Actual place, viz., at a.
For many astronomical observations an observatory
situated at one of the poles vk^oulJ be advantageous, and
for the one observer to directly note the position of the
two equatorial stars (Delta and Alpha) at the same time
a station so situated would be necessary ; indirectly,
however, the one astronomer could, we opine, observe
the locality of the star in the opposite longitude to his
station (which statfon we suppose to oe in the northern
hemisphere) with perfect or almost perfect precision. It
would be necessary to obtain the exact locality of the
pole of the celestial sphere according to the perpen-
dicular-axis theory •, having obtained this he would observe
the star Gamma when exactly on the meridian of his
station, and then continuing the meridianal line through
the place of the celestial pole, he would note one or more
stars on the produced meridian which would be within
the visible hemisphere when his station arrives in the op-
posite quarter (i. c, when his station has revolved through'
180"). Evidently he might thus find the place of a star
in the exactly opposite longitude to and in the same lati-
tude as that of the star Gamma, which would be the
place of Alpha. Supposing this method not to admit of
sufficient precision in practice, the earth's diurnal rotation
can be taken advantage of to observe the four stars suc-
cessively, namely four stars, respectively situated in or
near to the relative position we have indicated, are to be
observed successively at each of the six hours ; these
observations being repeated on the second day would
I
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I
I
PARALLAX OF THE STARS.
89
furnish the data whereby the precise relative place
(longitude) which each of the four stars would apparently
occupy, if viewed simultaneously, could be determined.*
Our computation (at the conclusion of Chapter II,) .
shows for a parallax of 20". 5, a distance of the star pro-
portionally about twice as great as the estimate given
by Herschel for a parallax of 1" only, f We will conclude
thest observations with the decided expression of opinion
that, when correctly ascertained, the parallax for the
nearer stars will be found to considerably exceed 20". t
Note. — Eeferring to page 49 — § (814) of quotation from
HerscheVa Outlines of Astronomy. ; ; ;
" The paper on parallax by Loi-d Wrottesley, ia Plnl.
Trans, for 1851, hero referred to, furnishes, as it seems to us,
very strong indirect evidence of the soundness of the per-
pendicular-axis theory. In consequence, according to our
view, of the non-recognition of the earth's vertical motion.
Lord Wrottesley finds unaccountable variations and apparent
discrepancies in observations of the same stars made with
groat care at different times. Eventually he coiicludos to
relinquish the attciiipt to obtain a decided parallax, .ground-
ing his resolution to do so, if we apprehend aright, mainly
on the apparently irregular and unsatisfactory character of
the results actually obtained.
* This last is tlie method wc havo already supposed to be made use of
in experimentally testing the reality of aberration by tri-monthl^ com-
parisons of the four equatorial stars.
t See quotation page 31, 5 (801). '"
X A helio-centric parallax of 1' would be (of course) equiralent to about
one-third of the distance represented by 20".4, and, according to our com-
putation, to about COO times the distance of the planet Saturn from tba
Sun.
o
: fV ■ ■'<;.
Xi- .'»;,)>•■■
SUPPLEMENTARY NOTE.
Heliocentric Parallax of the Earth and Planets.
The apparent path of the sun as it travels around the
celestial sphere in the undulatory path of the ecliptic, may
be considered ac the effect of heliocentric parallax upon the
sun itself (the parallax belonging to the horizontal motion of
the earth being, in this instance, compounded with that
belonging to its vertical motion : thus causing the oblique
position of the circle of the sun's apparent path).. For illus-
tration of this refer to Plate 12, Page 88 ; or, to Plate, Fig.
20, of Part Second.
The eai*th being on the eastern side of its orbit and
moving towards the west, the sun is seen on the western
side of the celestial sphere and appears to move towards the
east ; the earth having moved to the southern side of its
orbit, the sun is seen to the north ; the earth having arrived
at the western side of its orbit, the sun is seen on the
eastern side of the celestial sphere. The sun thus appears
to the terrestrial observer to move in the heavens from
west to east, or fVom east to west, and so on.
Now if we suppose the distance of the sun from the earth
to be increased 100 times or 1000 times, tho parallactic angle
would be thereby proportionally diminished, or, in other
woaIs, the apparent motion of the sun for the same actual
movement of the earth would be reduced in proportion to
I
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mm
92
HELIOCENTRIC PARALLAX.
the increase in the distance. We cannot, however, obtain
directly by observation the parallax of the sun resulting
from the movement of the earth through the* diameter of
its orbit, because, since the earth moves around the sun,
the effect is thereby greatly increased and becomes con-
tinuous, manifesting itself as an apparent semi-revolution
of the sun around the earth. But the geocentric parallax
of the sun having been correctly ascertained and the ma^-
nitudinal relation of the diameter of the earth's orbit to tho
diameter of the earth itself being known, wo possess the
means of readily determining by calculatation the helio-
centric parallax . <, . 1st. of the Earth itself ; 2nd. of each
of the other planets ; 3rd of any star at a definite known
distance from the sun.
To do this we only require to imagine that the earth
occupies at the same time two distinct places in its orbit,
fVom one of which the terrestrial observer viers the earth
itself at a distance of 90*^. The accompanying figure
(Fig. 14) will make perfectly clear this hypothetical
supposition, which as famishing a basis for the comparison
of the relative angles is not, we opine, open to objection.
* As 4000 : 190 millionB : : 8".6 : tang. 0/45" ; therefore
45° is the h. c. parallax of the earth.
(This result is obvious because the semi-diameter of the
earth's orbit equals the distance of the Sun, and the tangent
of 45° equals the radius.)
*> The geocentric parallax of the Bua and the distance of the sun from
the earth are immediatelj dependent each upon the other. so that if the
distance of the sun has been carrectljr ascertained to be a little more than
95 millions, it is certain that the geoceutric parallax of the sun is 8".6, and
vice vtna,
tBy this method, therefore, we find that a star of which the paral-
lax is ascertained to be 20". 4 would have a theoretical distance
from the sun of about 1000 times the distance of Saturn, instead
of 600 times, at which we have stated the estimate in the foot note
to page 69. •
HELIOCENTaiO PARALLAX. 98
Ilence, sinco the relative distances of the planets from the
sun arc approximately known, we may at once derive the
theoretical h. c. parallax of each planet fVom that of the
earth, Thus : Taking f
The Sun's distance at 95 million miles,
the h. c. parallax of the Earth 45* 0'
" Venus' distance at one-half that of the
Sun, the h. c. parallax of Venus 63** 26' 30"
" Mars' distance at twice that of the
Sun the h.c. parallax of Mars. 27<' 9' 36"
Tapiter's distance at 6 times that of
the Sun, the h.c. parallax of Ju-
piter. ..;. 110 21' 62"
" Saturn's at 10 times that of the Sun,
the h. c. parallax of Saturn 6<* 43' 12"
'f The distance of a Star at 10 times that
of Saturn, the h. c. parallax of
the Star 34' 23"
" of a Star at 100 times that of Saturn,
theh.c. parallax of the Star ... 3' 26"
" of a Star at 1000 times that of Saturn,
h. c. parallax of the Star 20". 6
" of a Star at 2000 times that of Saturn,
h. c. parallax of the Star 10". 3
" of a Star at 2,500 times that of Saturn,
the h. c. parallax of the Star ... about 8". 2
Now in this last quantity we obtain a convenient means
of testing and checking these distances by computation
based on an independent fact, because 8" .2 almost coincides
with the geocentric parallax of the Sun, which is 8" .6
Therefore: — As the semi-diameter of earth : semi-diameter
of earth's orbit : : semi-diameter of earth's orbit : the
distance of that Star of which the h. c. parallax coincides
with the g.c. parallax of the Sun; and accordingly . . ,
As 4000 : 95 millions : : 95 millions : 2375 times the
i
M
HILIOCINTEIO PAKALLAX.
distanc* of Saturn (or 23750 times the distance of the
Sun ;) Mrhich result is in close agreement with the pre-
ceding.*
We would suggest that parallactic observations of the
planets with the theoretical quantity iu each case as a guide
and a check on the apparent results, might be found a very
usefVil and desirable preparation for parallactic observations
of the stars.
* The diameter of the ptrallaoiic circle or tbg major axis of tliSi parallac-
tic ellipM would be (ofooune) twice at great aa the respective quantities
here giTea. The Star for example hariag 3'20 ' h. c. parallax, should bare
an extreme appareat motion, to and fro, of 6 02."
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APPENDIX.
TOE OBSERVED DEVIATION OF THE PLANET URANCS FROM
ITS (supposed) solar orbit.
t From HerscheVs Outluics of Astronomt/.
Plate 1, Fto. 4. — "The horizontal line, or abscissa, is
divided into eqiuil jjarts, each representing 50° of heliocentric
longitude in the motion of Uranus round the sun, and in
which the distiinces between the horizontal lines represent
each 100" of error in longitude. The result of each year's
observation of Uranus (or of the mean of all the observations
obtained during that year) in longitude is represented by a
black dot placed above or beU>w the point of the abscissa,
corresponding to the mean of the observed longitudes for
the year, above if the observed longitude be in excess of the
calculated, below if it fall short of it, and on the line if they
agree ; and at a distance from the line corresponding to this
difference, on the scale above mentioned. Thus, in Flam-
eteed's earliest observations in 1690, the dot so marked is
placed above the lino at 65".9 above the line, the observed
longitude being so much greater than the calculated."
(763.) " If, neglecting the individual points, we draw a
curve (indicated in the figure by a fine unbroken line)
through their general course, we shall at once perceive a
certain regularity in its undulations. It presents two great
elevations above, and one nearly as great intermediate de-
pression below the medial line or abscissa. And it is evident
that these undulations Avould bo very much reduced, and
the errors, in consequence, gi-catlj' palliated, if each dot
were removed in the vertical direction through a distance.
92
APPENDIX.
mid in the diioction iiuliciiLod by tho coiToJpoiidiii^poiii^t ot
ihG ctti'vo ABODE FGII, intorsocting tho abscissa at' points
li:0° distant, and making eqani oxcursions on oitbor side.
:}:**•* "
CiGi.) " Let us now consider tho effect of an erroneous
assumption of iho place of tho perihelion. Suppose in Fig. 2
X to represent tho longitude of a planet, and x y the
excess of its true above its moan longitude, due to ellii)-
ticity. * * * * "
(7CG.) " Let this increnso of jjcriod be made, and in cor-
respondence with that change let tho lonj^itudcs be reckoned
at a h, and the residual differences from that line instead of
AB, and wo shall have done all that can bo done in the
way of reducing and palliating these differences. "
The above quotation sufficiently explains the plate in
its application to our argument : namely, as indicating
the nature of the methods adopted for reconciling the
discordance between the theory and the observed facts.
For the full and more particular explanation of the plate,
the reader is referred to the work to which it belongs.
n
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