ASTRONOMY. 
■386 
however, a fatellite is feen pading over the difk like a dark 
fpot ; this has been attributed to fpots on the furface of 
the fatellite, and that the more probably, as the fame fa- 
tellite has been known to pafs over the difk at one time as 
a dark fpot, and at another time to be fo luminous, as..only 
to be didinguifhed from the planet at its ingrefs and egrefs. 
The periods or revolutions of Jupiter’s fatellites are 
found out from their conjunctions with that planet ; after 
the fame manner, as thole of the primary planets are dif- 
covered from their oppolitions to the Sun. And their dif- 
tances from the body of Jupiter, are meafured by a mi¬ 
crometer, and eftimated in femidiameters of that planet, 
and thence in miles. By the lated and mod exaft obfer- 
vations, the periodical times and didances of thefe fatel¬ 
lites, and the angles under which their orbits are feen from 
the Earth, at its mean didance from Jupiter, are as follow : 
\: 
| Satel- 
j lites. 
---■—— - 
Diltances in 
Angles of 
Orbit. 
Periodic Times. 
Semidia¬ 
meters. 
Miles. 
1 
2 
3 
4 
id. 18I1. 27'. 34" 
3 J 3 13 4 2 
7 3 4 2 36 
16 16 32 9 
5 $ 
9 sV 
1 4t 5 T 
2 5~xo 
266,000 
423,000 
676,000 
1,189,000 
3 ' 55 " 
6 14 
9 58 
17 30 
The eclipfesof the fiftellitesof Jupiter are of very great 
4ufe in adronomy. Fird, in determining pretty exactly the 
‘‘d-idaryce of Jupiter from the Earth. A fecond advantage 
dill more conftderable, is the proof which they give of the 
progredive motion of light. It is demondrated by thefe 
eclipfes, that light does not come to us in an indant, as 
the Cartefians pretended, although its motion is extremely 
rapid. For if the motion of light were infinite, or came 
to us in an indant, it is evident that we diould fee the com¬ 
mencement of aneclipfe of a fatellite at the fame moment, 
at whatever didance we might be from it; but, on the 
contrary, if light move progredively, then it is as evident, 
that, the farther we are from a planet, the later we (hall be in 
feeing the moment of its eclipfe, becaufe the light will take 
up a longer time in arriving at us ; and fo it is found in 
fact to happen, the eclipfes of thefe fatellites appearing al¬ 
ways later and later than the true computed times, as the 
Earth removes farther and farther from the planet. When 
Jupiter and the Earth are at their neared didance, being 
in conjunction both on the fame fide of the Sun, then the 
eclipfes are feen to happen the fooned ; and when the Sun 
is direCtly between Jupiter and the Earth, they are at their 
greated didance afunder, tIre didance being more than be¬ 
fore by the whole diameter of the Earth’s annual orbit, 
or by double the Earth’s didance from the Sun, then the 
eclipfes are feen to happen the lated of any, and later 
than before by about a quarter of an hour. Hence there¬ 
fore it follows, that light takes up a quarter of an hour in 
travelling acrofs the orbit of the Earth, or near eight mi¬ 
nutes in pafling from the Sun to the Earth ; which gives 
11s about 12,000,000 of miles per minute or 200,000 miles 
per fecond, for the velocity of light. A difcovery that 
waafird made by M. Roemer. 
The third and greated advantage derived from obferv- 
ing the eclipfesof the fatellites, is the know lege of the 
longitudes of the places on the Earth. Suppole twoob- 
fervers of fuch aneclipfe, the one, for example, at Lon¬ 
don, the other at fire Canaries; it is certain that the eclipfe 
will appear at the fame moment to both obfervers; but, as 
they are dtuated under different meridians, they count dif¬ 
ferent hours, being perhaps nine o’clock to the one, when 
it is only eight to the other ; by which obfervations of tire 
true time of the eclipfe, on communication, they find the 
difference of their longitudes to be one hour in time, which 
anfwers to fifteen degrees of longitude. 
The fydem of Jupiter and his fatellites is very large in 
itfelf; yet, on account of its immenfe didapce from us, it 
appears to occupy but a fmall fpace in the fphere of the 
darry heavens, and confequently every fatellite of Jupiter 
appears to us always near its primary, and to have an of- 
cillatory motion, like that of a pendulum, going alternately 
from its greated digreflion oifone fide of the planet, to its 
greated on the other, fometimes in a ftraight line, at others 
in an elliptic curve. When a fatellite is in its f’uperior 
femicircle, or that half of its orbit that is more didant 
from the Earth, its motion appears direCt to us ; when a 
fatellite is in its inferior femicircle, neared to the Earth, 
the apparent motion of it is retrograde. Both thefe mo¬ 
tions feem quicked, when the fatellite is neared the cen¬ 
tre of the primary, and dow er when they are more didant; 
at the greated didance they appear ftationary for a fhort 
time. Thefe fatellites mud afford a plealing fpeftacle to 
the inhabitants of Jupiter; for fometimes they will rile all 
together, fometimes be all together on the meridian-, 
ranged one under another, beddes their frequent eclipfes.. 
The mod didant fatellite, or that revolving in the outer- 
mod orbit, will appear to the inhabitants of Jupiter nearly 
as large as the Moon does to us; and from the four fateF- 
lites, the inhabitants of Jupiter will have four different 
kinds of months, and the number of them in their year 
not lefs than 4,500. 
Of SATUR N. 
Before the difcovery of the Georgian Sidtts, Saturn 
was reckoned the mod remote planet in our fydem ; he is 
expreded by Tj, the mark for lead, denoting an old man-, 
fupporting himfelf with a daff, reprefenting the ancient 
god Saturn. He diines but w*ith a pale feeble light, lei's 
bright than Jupiter and lefs ruddy than Mars. The us- 
informed eye imagines not, when it is directed to this little 
fpeck of light, that it is viewing a large and glorious 
globe, one of the mod dupendous of the planets. We 
need not, however, be lurprized at the vad bulk of Saturn* 
and its difproportion to its’ appearance in the heavens ; for 
we are to condder that all objeCts decreafe in their appa¬ 
rent magnitude, in proportion to their didance from us. 
This planet is perhaps one of the mod engaging objeCts 
that adronomy offers to our view ; it is 1'urrounded with 
a double ring, one without the other, and beyond thefe 
by feven fatellites, all in the plane of the rings ; tire rings 
and planets being all dark and denfe bodies, like Saturn 
himfelf, thefe bodies cading their diadows mutually one 
upon another ; though the reflected light of the rings is 
ufually brighter than that of the planet itfelf. Saturn 
was fufpeCted by Cadini and Fato, in 1683, to have a re¬ 
volution about its axis; for they one day faw a bright 
dreak, which difappeared the next, when another came 
into view near the edge of its difk ; thefe dreaks are called 
belts , like thofe of Jupiter. In 1719, when the ring dis¬ 
appeared, Callini faw its (hadow upon the body of the 
planet, and a belt on each fide parallel to the fhadovv. 
When the ring was vidble, he perceived their curvature 
was fuch as agreed with the elevation of the eye above 
the plane of the ring. He confidered them as fimilar to 
our clouds floating in the atmofphere; and, having a cur¬ 
vature fimilar to the exterior circumference of the ring, 
he concluded that they ought to be nearly at the fame dis¬ 
tance from the planet, and confequently the atmofphere 
of Saturn extends to the ring. Dr. Herfchel found that 
the arrangement of the belts always followed the direction 
of the ring; thus, as the ring opened, the belts began to 
fhew an incurvature anfwering to it. And during his ob¬ 
fervations on June 19, 20, and 21, 1780, he faw the fame 
fpot in three different fituations. He conjeCtured there¬ 
fore, that Saturn revolved about an axis perpendicular to 
the plane of its ring. The truth of his conjecture he has 
now verified, having determined that Saturn revolves 
about its axis in ioh. 16'. 0-4". Phil. Tranf. 1794. The 
rotation is according to the order of the figns. 
Concerning the figure of Saturn; we find that Galileo 
fird perceived that his figure is not round; but Huygens 
(hewed that this-was owing to the pofitions of his ring; 
for his fpheroidical form could only be feen by Herfchel’s 
telefcope ; though indeed Cadini, in an obfervation made 
June 
