ASTRONOM Y. 
3S7 
Saturn. M. tic la I.ande makes it 5f. o°. 27'. From the 
obfervation of M. Bernard at Marfeilles in 1787, it ap¬ 
pears, that the node of this fatellite is retrograde. Dr. 
Halley difcovered that the orbit of the fourth fatellite was 
eccentric. For, having found its mean motion, lie difco¬ 
vered that its place by obfervation was at one time three 
degrees forwarder than by his calculations, and at other 
obfervations it was 2 0 30' behind ; this indicated an eccen¬ 
tricity ; and he placed the line of the apfides in iof. 22 0 . 
M. Caffini obferved, that the fifth fatellite difappeared 
regularly for about half its revolution, when it was to the 
end of Saturn ; from which he concluded, that it revolved 
about its axis; he afterwards however doubted of this. 
But Sir Ifaac Newton, in his Principia, concludes from 
hence, that it revolves about its axis, and in the fame time 
that it revolves about Saturn; and that the variable ap¬ 
pearance arifes from fome parts of the fatellite not reflect¬ 
ing fo much light as others. Dr. Herfchel has confirmed 
this, by tracing regularly the periodical change of light 
through more than ten revolutions, and finding it, in all 
appearances, to be cotemporary with the return of tire fa¬ 
tellite to the fame (ituation in its orbit. This is farther 
confirmed by fome obfervations of M. Bernard at Mar¬ 
feilles, in 1787; and is a remarkable indance of analogy 
among the fecondary planets. 
Thefe are all the latellites which were known fo revolve 
about Saturn till the year 1789, when Dr. Herfchel, in a 
paper in the Phil. Tranl. for that year, announcedthe dif- 
covery of a fixth fatellite, interior to all the others, and 
promifed a farther account in another paper. But, in the 
intermediate time, lie difcovered a feventh fatellite, inte¬ 
rior to the fixth ; and, in a paper upon Saturn and its ring, 
in the Phil. Tranf. 1790, he has given an accuunt of the 
difeovery. After his obfervations upon the ring, he fays, 
he cannot quit the fubjeft without mentioning his own lur- 
mifes, and that of feveral other adronomers, of a fuppofed 
roughnefs of the ring, or inequality in the planes and in¬ 
clinations of its flat (ides. This fuppofition arole, from 
feeing luminous points on its boundaries projecting like the 
Moon’s mountains ; or from feeing one arm brighter or 
longer than another; or even from feeing one arm when 
the other was invifible. Dr. Herfchel was of this opinion, 
till he favv one of thefe points move off the edge of the 
ring in the form of a fatellite. With his 20-feet tele- 
fcope he fufpefted that he faw a fixth fatellite; and, oa 
Augud 19, 1787, marked it down as probably being one ; 
and, having finifhed his telefcope of forty feet focal length, 
he faw fix of its fatellites the moment he directed his te¬ 
lefcope to the planet. This happened on Augud 28, 1789. 
The retrograde motion of Saturn was then nearly 4' 30" 
in a day, which made it very eafy to afeertain whether the 
Bars he took to be fatellites were really fo; and, in about 
two hours and an half after, he found that the planet had 
vifibly carried them all away from their places. He con¬ 
tinued his obfervations, and, on September 17, he difco¬ 
vered the feventh fatellite. Thefe two fatellites lie wit Iv¬ 
in the orbits of the other five. The orbits of thefe fatel¬ 
lites lie fo near to the plane of the ring, that the difference 
cannot be perceived. 
As foon as Dr. Herfchel had made obfervations fufficient 
to conflruct Tables of their mean motions, lie calculated 
their places backwards, and found that his fufpicions of 
the exidence of thefe fatellites, in the fhape of protube¬ 
rant points on the arms of the ring, were confirmed. He 
frequently faw the fird, fecond, third, fourth, and fifth, fa- 
tellites, pafs before and behind the ring in Inch a manner, 
that they ferved as an excellent micrometer tomeafure its 
thicknefs by. It may be proper to mention a few indances, 
as they ferve alfo to folve fome phenomena obferved by 
other adronomers, without having been accounted for in 
any manner that could be admitted confidently with other 
known faCts. July 18, 1789, at 19I1. 41'. 9 ". fiderial time, 
the third fatellite feemed to hang upon the following arm, 
declining a little towards the north, and was feen gradually 
to advance upon it towards the body of Saturn; but the 
Vol. II. No. 78. 
ring was not fo thick as the lucid point. July 23, at 19b. 
4 1 '• 8''. the fourth fatellite was a very little preceding the 
ring, but the ring appeared to be lefs than half the thick- 
nefe of the fatellite. July 27, at 20b. 15'. 12". the fourth 
fatellite was about the middle, upon the following arm oi 
the ring, and towards the fouth ; and the fecond at the 
farther end, towards the north; but the arm was thinner 
than either. Augud 29, at 22I1. (2'. 25". the fifth fatel¬ 
lite was upon the ring, near the end of the preceding arm, 
and the thicknefs of the arm feemed to be about one-third 
or one-fourth of- the diameter of the fatellite, which, in 
the fituation it then was, he took to be lets tharrone-fecond 
in diameter. At the fame time, the fird appeared at a 
little didance following the fifth, in the fhape of a bead 
upon a thread, projecting on both (ides of the fame arm. 
October 16, he followed the fird and fecond latellites up 
to the very ditk of the planet ; and the ring, which was 
extremely faint, did not obdrndt his feeing them gradually 
approach the dilk. Thefe obfervations lhew the extreme 
thinnefs of the ring. But Dr. Herfchel farther obferves, t 
that there may be a refraction through an atmofphere of 
the ring, by which the fatellites may be lifted up and de- 
preffed, fo as to become vifible on both iides of the ring, 
even though the ring fhould be equal in thicknefs to tl;C 
fmalled fatellite, which may amount to roao miles. 
The periodical revolutions, and didances of thefe fate} - 
lites from the body of Saturn, ex preffed in femidiameters 
of that planet, and in miles, are as follow ; 
Diftances in 
Satel¬ 
lites. 
Periods. 
Semidi¬ 
ameters 
j 
I Miles. 
Diam. of 
Orbit. 
I 
id. 
2ih. 18' 
• 27" 
4 * 
170,000 
i' 27" 
2 
2 
J 7 
41 
22 
si 
217,000 
1 5 2 
3 
+ 
I 2 
25 
12 
8 
303,000 
2 36 
4 
1 
22 
4 » 
*3 
18 
704,000 
6 18 
5 
79 
7 
48 
0 
54 ' 
2,050,000 
i 7 4 
6 
1 
8 
53 
9 
s 4 
t35, otx > 
1 «4 
7 
0 
22 
40 
46 
2 *- 
107,000 
0 57 
There is reafon to believe that all the fatellites both of 
Jupiter and Saturn, revolve about their axes; for the fa¬ 
tellites of the former appear at different times to be of very 
different magnitudes and brightnefs. The fifth fatellite 
of Saturn was obferved by M. Caffini for feveral years as 
it went through the eadern part of its orbit to appear lefs 
and lefs, till it became invifible; and in the wedern part 
to increafe again. Thefe phenomena can hardly be ac¬ 
counted for, but by fuppoling fome parts of the furfaces 
to be unfit to refled: light; and therefore, when fuch parts 
are turned towards the Earth, they appear to grow lefs, 
or to difappear. As the fame appearances of this fatel¬ 
lite returned again when it came to the fame part of itS-- 
orbit, it affords an argument that the time of the rotation 
about its axis is equal to the time of its revolution about 
its primary, a circumftance (imilar to the cafe of the Moon 
and Earth. See Dr. Herfchel’s account of this in the Phil. 
Tranf. 1792. The appearance of this fatellite of Saturn 
is not always the fame, and therefore it is probable that 
the dark parts are not permanent. 
On the Secular Motions of J upiter and Saturn, 
The time of a revolution of Saturn deduced from the 
modern obfervations, comes out greater than that deduced 
from a comparifon of the modern with the ancient obfer* 
vations. If therefore the modern obf^ryations could be 
depended upon to give the time of a revolution nearer than 
that difference, it would prove that the length of Saturn’s 
year is increafing. Now, although obfervations made at. 
a finall interval of time could not be fufficient to eftablift 
this point, yet, from a comparifon of our obfervations with 
thofe made by Tycho, it appears that this is the cafe. The 
length of the year, therefore, when afeertained for one 
time, will afterwards want a correction ; and !#ie quantity 
5 G of 
