Royal Society (Great Britain).

Philosophical magazine online

. (page 25 of 59)
Online LibraryRoyal Society (Great Britain)Philosophical magazine → online text (page 25 of 59)
Font size
QR-code for this ebook

sity of stars in this zone, according to the numbers above stated, is
5906 to a field.

" A zone of 3° in breadth bisected throughout by the galactic
cirde, or extending 14° on its north and as much on its south side,

Phil. Mag. S. S. Vol. S3. No. 221. Sept. 1848. R

Digitized by VjOOQ IC

254 Notices respecting Nefo) Books.

was found to include 84 gauges containing 6258 9tan, giving an
average of 74*50 stars to a field. The average would have been
much higher if, instead of following the course of this circle, a zone
of equal breadth pursuing the irregular line of maximum intensity of
the Milky Way had been chosen, which in some places deviates by
several degrees from the great circle which expresses its general
situation. Judging from the course of the counted gauges only, the
mean density of stars in the medial line of the actual galaxy in that
part I have observed would be somewhere about 90 stars to the field,
but this must be considered as exclusive of the more densely clut*
tering masses." — ^P. 380.

On the northern side of the galactic circle the average number of
stars to a field was found to be as follows : — In the zone extending
from 0° to 15°, 51-28 ; from 15° to 30°, 2347 ; from 30° to 45^
14*46 ; from 45° to 60°, 7*7 1. The number of gauges taken in those
zones were respectively 321, 195, 68 and 21.

" Nothing," says Sir John Herschel, " can be more striking than
the gradual but rapid increase of density on either side of the Milky
Way as we approach its course, and the reproduction of nearly the
same law of graduation on the north side which holds good on the
south, so far as the comparative paucity of the gauges taken in that
direction allow us to judge. On the whole, this induction, founded
as it is on the actual enumeration of 68948 stars contained in 2299
fields, must be admitted as decisive of the specific point in question,
and as completing the evidence to the same effect afforded by Sir
William HerscheTs observations in the northern hemisphere."

Calculating upon the above averages the number of stars visible
enough to be distinctly counted in the 20'foot reflector over the whole
sphere, it will be found to be 5,331,572, or somewhat lees than five
and a half millions. ** That the actual number is much greater there
can be little doubt, when we consider that large tracts of the Milky
Way exist so crowded as to defy counting the gauges, not by reason
of the smallness of the stars, but their number."

It will be understood that great local departures from the law of
distribution above indicated occur in all regions, and nowhere more
remarkably than in the Milky Way itself, whose irregularities of
breadth and structure are most conspicuous and singular ; but with
the exception of portions of the galaxy, nothing was found in any
part of the heavens meriting in the smallest degree to be regarded
as systematic, as respects those deviations from perfect regularity.
So purely local are they, that on a careful revision of the whole chart
Sir John found it difficult to specify any considerable areas over
which an average density of stars prevails materially differing from
what might be expected from the law above indicated, regarded as a
function of the galactic polar distance.

Another interesting question remains, namely, whether the in-
creased frequency of stars in approaching Uie Milky Way is observable
in respect of stars of all classes of magnitude indifferently. In order
to ascertain this point, not only the total numbers of stars were set
down in counting the gauges, but those of all the several magnitudea

Digitized by VjOOQ IC

Notices respecting Nevo Books. 255

down to the- eleventh inclusive ; and a table is given in which ave
exhibited the numbers of all the stars of the several magnitudes
occurring in the registered gauges, distributed in the respective zones
of galactio polar distance. The following are the results : —

" On a general view of the table >t appears that the tendency to
greater frequency, or the increase of density in respect of statistical
distribution, in approaching the Milky Way, is quite imperceptible
among stars of a higher magnitude than the 8th, and except on the
very verge of the Milky Way itself, stars of the 8th magnitude can
hardly be said to participate in the general law of increase. For the
9th and 10th, the increase, though unequivocally indicated over a
cone extending at least S0° on either side of the Milky Way, is by
no means striking. It is with the 11th magnitude that it first be-
comes conspicuous, though still of small amount when pompared
with that which prevails among the mass of stars of magnitudes
inferior to the 11th, which constitute sixteen-seventeenths of the
totality of stars within 30° on either side of the galactic circle.

"Two conclusions seem to follow inevitably from this; viz. — Ist,
that the large stars are really nearer to us (taken en masse, and with-
out denying individual exceptions) than the smaller ones . . • . ; 2nd*
that the depth at which our system is plunged in the sidereal stratum
constituting the galaxy, reckoning from the southern surface or
limit of that stratum, is about equal to that distance which, on a
general average, corresponds to the light of a star of the 9th or 10th
magnitude, and certainly does not exceed that corresponding to the
Jlth/'—P. 383.

The 2nd Section of this chapter contains a minute and exceedingly
interesting description of the general and telescopic appearance of
the Milky Way — too long, however, to be extracted here in extenso,
end scarcely susceptible of abridgement. We must make room for
two or three paragraphs.

" Immediately after the contraction between X Centauri and ^ Cruets
tiie Milky Way suddenly expands so as to include the southern half
of the cross and the northern portion of Muscoy whence it proceeds
to P Centmiri, embracing in this wide expansion that singular vacuity
on the south following side of the cross, called the ' Coal-sack,' a
pear-shaped oval, whose greatest length is about 8°, and breadth 5°,
^e longer axis being nearly parallel to the line joining a and fi Crucis,
which hne is very nearly a tangent to the north-preceding portion
of its circumference. As this is always regarded by voyagers and
travellers as one of the most conspicuous features of the southern
sky, it may not be iiTclevant to state a few particulars as to its tele-
scopic constitution. It is by no means entirely devoid of stars, the
lowest gauges being 9 and 7, and no blank fields being specified as
occurring in it. The cluster of telescopic stars h. 3407 is actually
contained within its area; and even in the middle of its extent
gauges of 29 and 48 stars are noted. Its striking blackness is,
therefore, by no means owing to an absolute want of telescopic stars,
but rather to its contrast with the very rich portion of the Milky
Way a4iacent> where the gauges run up to 98, 100, 120, and even


Digitized by VjOOQ IC

286 Notices respecting New Books.

200 on the preceding side-^to 65, 108, 108, on the following; 68
on the south, and 90 on the north, llie contrast is enhanced by
the suddenness of the transition." — P. 384.

" It is about this region, or perhaps somewhat earlier, in die in-
terval between ii Argds and a Crucis, that the galactic circle, or
medial line of the Milky Way, may be considered as crossed by that
of the zone of large stars which is marked out by the brilliant con-
stellation of Orion, the bright stars of Canis Major, and almost all
the more conspicuous stars of Argo, the Cross, tbe Centaur, Lupus,
and Scorpio. A great circle passing through t Ononis and a Crucis
will mark out the axis of the zone in question, whose inclination to
the galactic circle is therefore about 20^, and whose appearance
would lead us to suspect that our nearest neighbours ill the sidereal
system (if really such), form part of a subordinate sheet or stratum
deviating to that extent from parallelism to the general mass which,
seenjpt-ojected on the heavens, forms the Milky Way." — P. 385.

** Bode's, and most other celestial charts, make the Milky Way
bifurcate, in Cygnus and the tail of Scorpio, into two great streams,
both of which, the preceding and the following, preserve their conti-
nuity, from point to point, unbroken. This, however, is the case
only with the following, or main stream, whose course we have just
traced. The preceding is discontinuous. Its northern portion (from
Cygnus southwards) terminates precisely at the equator, just beyond
the bisection of a line ri Serpentis and j3 Ophiuchi ; and from this
to the nearest point of its southern continuation (if it can be so con-
sidered) there is a break of 14^ in extent to the star o Serpentis,
totally devoid of all appearance of it."

" If we now consider the telescopic structure of the region spread
over by, and inclosed between the effusions of the Milky Way in the
body and tail of Scorpio, the hand and bow of Sagittarius, and the
following leg of Ophiuchus, we shall find it, in the highest degree,
interesting and complex. No region of the heavens, in fact, isfriUer
of objects beautiful and remarkable in themselves, and rendered still
more so by their mode of association, and by the peculiar features
assumed by the Milky Way, which are without a parallel in any
other part of its course." — P. 386.

The description is wound up with the following remarks : —

" From the foregoing analysis of the telescopic aspect of the Milky
Way in this interesting region, I think it can hardly be doubted that
it consists of portions differing exceedingly in distance, but brought
by the effect of projection into the same or nearly the same visual
line ; in particular, that at the anterior edge of what we have called
the main stream, we see, foreshortened, a vast and illimitable area
scattered over with discontinuous masses and aggregates of stars in
the manner of the cumuli of a mackerel sky, rather than of a stratum
of regular thickness and homogeneous formation ; and that in the
inclosed spaces insulated from the rest of the heavens by the prece-
ding and following streams, and the ' bridges ' above spoken of as
connecting them (as, for instance, in that which includes XScorpii),
we are, in fact, looking out into space through vast chimney-form

Digitized by VjOOQ IC

Royal Astronomical Society. 237

or tabular vacancies whose terminations are rendered nebulous by
the effect of their exceeding distance, and at the same time are
brought by that of perspective to constitute the interior borders of
the apparent vacuities. It is possible that the globular clusters we
eee scattered over it are nothing more than such masses in a higher
state of aggregation, to which perhaps the others are by slow degrees
advancing. Yet in that case we should certainly be prepared to ex-
pect specimens of an intermediate character to occur in considerable
numbers, scattered among them, whereas, in fact, it would be diffi-
cult to particularize any objects in the region in question which can
be quite fairly so considered. The intermediate stages of central
condensation between the highly compressed globular cluster, and
the dilute and nearly uniform nebuloid patch, if not altogether want-
ing, are, at all events, feebly represented." — P. 390.

In the drd Section mention is made of a phsenomenon sometimes
observed in the course of sweeping, which seems to indicate the exist-
ence of starry regions of great extent and excessive remoteness, not
in traceable connexion with the Milky Way, except in some parti-
cular localities, yet possibly outlying portions of that system. It
consists in " an exceedingly delicate and uniform dotting or stippling
of the field of view by points of light too small to admit of any one
being steadily and fixedly viewed, and too numerous for counting
were it possible so to view them." Sir John states that he always
felt satisfied of the reality of the pheenomenon at the moment of
observation ; but the conviction was not permanent, the idea of illu-
sion continually arose subsequently, partly from the extreme delicacy
of the points of light, but chiefly from the circumstance, that when
noticed it was almost invariably soon alter the commencement of the
sweeping. But he thought it right, notwithstanding the doubt, not
to suppress all mention of the pheenomenon, " which, if it arise from
physiological causes, is at least curious and remarkable as a case of
optical iUusion." A list is given of the places in the heavens where
it was remarked, and another of places where the ground of the sky
was recorded as particularly black, and certainly devoid of any such
stippling or nebidous appearance ; and some details are added with
the view of throwing further light on the subject.

The chapter closes with a reference to one of the plates, repre*
senting the course and aspect of the Milky Way from Antinous to
Monooeros, delineated with as much precision as the nature of such
work admits, that is, with the naked eye by faint lamp-light in the
open air. The differences in some very obvious features between
this and former representations of the same portion of the heavens
are sufficiently remarkable.

XXXVI. Proceedings of Learned Societies.

[Continued from p. 162.]
March 10, /^N the Interior Satellites of Uranus. By the Rev. W.
1848, V/ R.Dawes.
In the Monthly Notices for January last were printed some obser^

Digitized by LjOOQIC

3S6 Royal Astronomical Society.

vaidons of an interior satellite of Uranus, which had been made in
the autumn of last year by Mr. Lasseli and M. Otto Struve. The
results are in Beveral respects interesting and remarkable. The fact
that one observer always saw the close satellite on the northern side
ai the planet only, while the other as uniformly observed it only on
tiie southern side, is sufficiently curious to invite further investigation.
It is however obvious, that the observations at Starfield and at
Poulkova are utterly incompatible with each other. While the latter
point to an approximate period of 3^ 22^^ 10™, the period indicated
by the former is only about 2^ 2^ 43°** 6. The distance also of the
satellite carefully estimated by Mr. Lasseli on Nov. 6, 1847, under
ftivourable circumstances and with great probability of considerable
accuracy, was only IT'; the position-angle being estimated 349^.
Now, assuming the direction of the major axis of the projected orbit
to be from 10® to 190°, as determined by M. O. Struve for the satel-
lite observed by him ; and assuming also that the apparent elllpticity
of the orbit does not greatly differ from that of the orbits of the
bright satellites I. and II. ; we find that the distance of the satellite,
at its greatest elongation, would be 12'^' 2, on the supposition that
the distance was correctly estimated at 11" when the position-angle
was 349°. But this is almost precisely the greatest elongation
theoretically due to a satellite revolving about Uranus in the period
indicated by Mr. lAssell's observations. We are thus led to the
conclusion, that there are at least two satellites interior to the nearest
bright one : and to avoid the confusion which might arise from ap*
plying numbers of any kind to the smaller satellites, I beg permission
to denominate them for the present, a, b, c, &c. in the order of di-
stance from the primary ; — a being the satellite (Observed by Mr. Las-
seU, and c the satellite observed by M. O. Struve.

M. O. Struve suggests that the satellite observed by him may lose
much of its light when in the northern portion of its orbit ; and this
may be the reason why Mr. Lasseli did not see it on Nov. 6, 1847,
on which favourable night c must have been near its greatest north-
em elongation. On all the other nights when a was observed by
Mr. Lasseli, c was very close to the planet, with one exception only,
on Sept. 14, at which time c was near its greatest elongation south*
wards, and might perhaps have been seen if the night had been suf-
ficiently good. Neither Mr. Lasseli, however, nor myself then ob-
serving with him, perceived any such object in that place.

It should here be stated, that the estimated position for Sept. 14,
as given in the printed table, appears to be erroneous. It is incon*
sistent with the diagrams independently made at the time by Mr«
Lasseli and myself, which, taking the measured position of II. as a
guide, show that the position of a was about 80° north preceding ; —
whereas the angle as printed is 80° north following. It seems clear,
therefore, that the angle should be 350° instead of 10°. This being
rectified, and the position-angles computed for the times of observa-
tion, by reckoning back from the peculiarly valuable observation of
Nov, 6 as an epoch, and assuming a period of 2*' 2^ 43°** 6, the esti-
mated angle minus the calculated angle comes out, for Sept. Hss - 2°;
lor Sept. 27 =-2°; for Sept. 29 =2 + 1°; for Oct. 1 = 4-2°..

Digitized by VjOOQ IC

Royal Astronomical Society. f^89

That the satellite a should never have heen seen by M. O. Struve
may arise from its becoming faint in the southern portion of its
orbit, as c probably does in the northern. It appears, from calcula-
tion on the assumptions before mentioned, that on every night when
M. O. Struve saw c« with the only exception of Dec. 10, a was in
the southern portion of its orbit ; yet sufficiently distant from his
observed position of c to render any confusion between them impos-
sible. On Nov. 28, indeed, the position-angles of the two satellites
must have been almost precisely tne same : but the measured distance
of c was 16"- 85 ; while the distance of a could not have exceeded
1 1'' in that part of its orbit. It should be noticed, with reference
to the non-observation at Poulkova of the satellite a on Dec. 10, that
diough on that night near its greatest northern elongation, it was at
a smaller distance from the primary than c ever was when it was
observed. Unless, therefore, the night were unusually fine, so faint
an object might easily be overlooked.

So long ago as the autumn of 1845* Mr. Lassell occasionally saw
a faint object, supposed to be a satellite, at about the same distance
from the planet as a was observed to have been last year ; and, with
only one exception, it was always seen on the northern side of the
primary, and usually in the north preceding quadrant, in which it
was uniformly seen in 1847. On Oct. 5, 1845, at 12** 26" Green-
wich mean time, being on a visit at Starfield, I had the gratification
of seeing this satellite in the 20«foot reflector. When the light of
the planet was hidden from the eye by a bar, the satellite became
steadily visible ; and a careful diagram being made, both by Mr.
LABSell and myself independently, the position-angles deduced from
them agreed within 3^, the mean being 324°* 1 . 'Phis is very nearly
the same with the estimated position on Sept. 27, 1847; and assu-
ming, that in the interval of 721*8896 days, 342 complete revolutions
had been performed, the period comes out 2^ 2^ 39"* 36". It seems
probable, therefore, that the period of this satellite does not differ
mnch from that quantity.

Supposing that those nights on which the satellites I. and II. were
measured at Starfield in 1 847 were probably favourable, though a
was not noticedt it becomes interesting to know whether a were
then at such a distance from the planet as might allow it to be visible
in the 20>foot reflector. I have therefore computed the places of a
for each of those nights ; and the results show, that only on Oct'. 16
was it in the northern portion of its orbit, and not very close to its
primary. On every other occasion, therefore, it was probably either
invisible in the southern portion of its orbit» or overpowered by its
vicinity to the planet.

By a similar computation, it becomes evident, that on none of
those nights was the satellite c near its greatest southern elongation;
and that, therefore, it was probably invisible on the northern side of
the planet, or too near it to be discerned.

It is singular, that in the whole series of observations at Starfield
and Poulkova, only one night, Nov. 1, is common to them both ;
nd that was^f so indifierent a quality at Starfield, as to render it

Digitized by VjOOQ IC

240 Royal Astronomical Society*

improbable that so difficult an object aa the satellite c could have
been detected; a was then in the south following quadrant, and
rather near the planet, so that it was not seen at either place of ob-

On the whole, therefore, it seems highly probable that the interior
satellites observed by Mr. Lassell and M. O. Strove are entirely
distinct ; that the one becomes invisible in the southern part of its
orbit, and the other in the northern ; and that, during the last appa-
rition of Uranus, it has so happened, that each satellite has been seen
only by its own observer. If either of them is the same as was dis-
covered by Sir W. Herschel, it seems most likely to be that observed
at Poulkova.

The object observed at Starfield on Nov. 6, 1847, and supposed
to be an intermediate satellite between I. and II., was estimated to
be only 10'' distant from the planet. But its position was then
almost precisely opposite to that of I., whose distance was estimated
at 20". It is therefore obvious, that if this were a satellite, its orbit
must be interior to that of I. It could not however have been c,
which must at that time have been near its greatest northern elon-
gation, and therefore probably invisible. Moreover, its greatest
distance could be only about 15"^^; and it would therefore be nearly
intermediate between a and c, and for the present it may be distin-
guished as b. Though it may seem premature to attempt any con-
clusions from a single observation of so difficult an object, yet, on
the other hand, the observation of Nov. 6 was peculiarly worthy of
reliance. The night was unusually fine. UTie planet was viewed
for two hours, during which time the supposed satellites were carried
along with it. The distance also of b was estimated just half that
of I., which was almost exactly opposite to it. And that the esti-
mated distance of I. (20'^) was nearly correct, appears from comparing
it with the measured distance of the same satellite (20''*57 ) on Oct. 1 1 ,
when it was in almost precisely the same part of its orbit. If there-
fore it were really a satellite, it appears probable that there are three
satellites revolving within the orbit of I., at apparent mean distances
of about 12^ 15", and 18".

Mr. Lassell having favoured me with a communication of all his
observations of a close satellite of Uranus, with the position-angles
estimated at the time, and copies of the diagrams, it appears from
them, that only on one occasion has such an attendant been un-
doubtedly seen on the southern side of the planet. This occurred
on Sept. 27, 1845, at 12"* 6" Greenwich mean time. The estimated
angle was 160°, the distance was three diameters of the planet from
the edge of the disc, which gives 14" for the central distance. This
does not at all agree with any probable period of the satellite a.
But on calculating the period of c from this observation, compared
with each set obtained by M. O. Strove in 1847, employing as a
guide to the number of entire revolutions the approximate period
deduced by him, viz. 3*^ 22*» 10", and giving a weight to the Poul-
kova observations proportional to the number from which each result
is derived, the period comes out from the mean of the whole,— 3^

Digitized by VjOOQ IC

Rai/al Astronomical Society. S41

22^ 8*" 35*. Mr. lAssell's estimation of distance corresponds to a
greatest elongation of about 1 1" ; which, as well as the period, agrees
so nearly with the result of the Poulkova observations, as to render
it highly probable that the satellite observed by Mr. Lassell on
Sept. 27, 1845, is the same as that observed by M. O. Struve.

Extract of Letter from M. Otto von Struve to the Astronomer

" Yoa will see from the Astronomische Nachrichten, &c., that we
have not been idle at Poulkova since your visit. In addition to the
published accounts I have little to say, except that my father's cal-
culations of the great Russian meridian arc give a considerably larger
value to the difference between the two axes of the earth than has
been hitherto found. I cannot tell you the exact quantity, as the
calculations are not completed.

'* I have finished my observations of the satellites of Uranus for

Online LibraryRoyal Society (Great Britain)Philosophical magazine → online text (page 25 of 59)