ASTRONOMY. 
heat near the equator, which is parallel to the belts and 
conrfe of the fpots. Several large fpots, which appear 
round at one time, grow oblong by degrees, and then di¬ 
vide into two or three round fpots. From thefe fpots, the 
time of the rotation of Jupiter can be very accurately af- 
certained. M. Caflini found the time of the rotation to 
be 9I1. 56'. from a remarkable fpot which he obferved in 
the year 1665. In October 1691, he obferved two bright 
fpots almoft as broad as the belts ; and at the end of the 
month he faw two more, and found them to revolve in 
911.51'. he alfo obferved fome other fpots near Jupiter’s 
equator, which revolved in 9I1. 50'. M. Maraldi, from 
a great many obfervations of the fpot obferved by Caf- 
fini in 1665, found the time of rotation to be 9I1. 56'. 
and concluded that the fpots had a dependence upon the 
contiguous belt, as the fpot had never appeared without 
the belt, though the belt had without the fpot. It conti¬ 
nued toappear and difappear till 1694,and was not feenany 
more till 1708 ; hence he concluded, that the fpot was 
fome effufion from the belt upon a fixed place of Jupiter’s 
body, for it always appeared in the fame place. Dr. 
Herfchel found the time of rotation of different fpots to 
vary ; and that the time of revolution of the fame fpot di- 
minifhed ; for the fpot obferved in 1778 revolved as fol¬ 
lows. From Feb. 25, to March 2, in 9I1. 55'. 20". from 
March 2 to 14, in 9h. 54'. 58". from April 7 to 12, in 9I1. 
51'. 35". Alfo, from a fpot obferved in 1779, ' ts rotation 
was, from April 14to the 19th, in 9I1.51'.45". from April 19 
to the 23d, in 9b. 50'. 48".' This, he obferves, is agreeable 
to the theory of equinoctial winds, as it may be fome time 
before the fpot can acquire the velocity of the wind ; 
and, if Jupiter’s fpots fliouldbe obferved in different parts 
of its year to be accelerated and retarded, it would amount 
almoft to a demonftration of its monfoons, and their peri¬ 
odical changes. M. Schroeter makes the time of rotation 
ph. 55'. he obferved the fame variations as Dr. 
Herfchel. The belts of Jupiter are generally parallel to 
its equator, which is very nearly parallel to the ecliptic ; 
they are fubjeCJ to fo many variations, both in refpect to 
their number and figure, that it is probable they do not 
adhere to the body of Jupiter, but exift in its atmofphere. 
The axis of Jupiter is fo nearly perpendicular to his or¬ 
bit, that he has no fenlible change of feafons ; which is a 
great advantage, and wifely ordered by the Author of na¬ 
ture. For, if the axis of this planet were inclined any 
confiderable number of degrees, juft fo many degrees 
round each pole would in their turn be almoft fix years to¬ 
gether in darknefs. And, as each degree of a great circle 
on Jupiter contains about 706 miles, it is eafy to judge what 
'vaft traCls would be rendered uninhabitable by any confi- 
•derable inclination of his axis. 
The difference between the equatorial and polar diame¬ 
ters of Jupiter, is upwards of 6000 miles ; the former 
being to the latter as 13 to 12 : fo that his poles are more 
titan 3000 miles nearer his centre than the equator is. This 
happens from his quick motion round his axis ; for the 
fluids, together with the light particles, which they can 
carry or waflt away with them, recede from the poles which 
are at reft, towards the equator where the motion is quick- 
eft, until there be a futfieient number accumulated to 
make up the deficiency of gravity toft by the centrifugal 
force, which always arifes from a quick motion round an 
axis : and, w hen the deficiency of weight or gravity of tire 
particles is made up by a fufficient accumulation, there is 
then an equilibrium, and the equatorial parts rife no higher. 
Jupiter’s orbit is i° 20' inclined Xo the ecliptic. The 
place of his aphelion 9 0 10' of Libra; the place of his af- 
cending node 7 0 29' of Cancer; and that of his fouth or 
defeending node 7 0 29' of, Capricorn. The eccentricity 
of his orbit is fa of his mean diftance front the Sim. The 
Sun appears to Jupiter but the 48th part fo large as to us ; 
and his light and heat are in the fame fmall proportion ; 
but this is compenfated by the fliortnefsof his nights, and 
by four moons or fatellites, fome of them nearly as large 
as eur Earth, which revolve about him ; fo, that there is 
Vol. II. No. ?8. 
385 
fcarcely any part of this huge planet but what is during the 
whole night enlightened by one or more of thefe moons, 
except his poles, whence only the fartheft Moons can be 
feen, and where their light is not wanted, bccaufe the Sun 
conftantly circulates in or near the horizon, and is very 
probably kept in view of both poles by the refraftion of 
Jupiter’s atmofphere, which, if it be like ours, has cer¬ 
tainly refractive power enough for that purpofe. This 
planet, feen from its neareft Moon, appears 1000 times as 
large as our Moon does to us; increafing and waning in all 
her monthly fltapes every 42^ hours. 
Though there be four primary planets below Jupiter, 
yet an eye placed on his furface would never perceive any 
of them ; ttnlefs perhaps as fpots palling over the Sun’s 
difk, when they happen to come between the eye and the 
Sun. The parallax of the Sun, viewed from Jupiter, will 
fcarcely be fenlible, being not much above twenty feconds ; 
and tlie Sun’s apparent diameter in Jupiter, but about fix 
minutes. Dr. Gregory adds, that an aftronomer in Jupi¬ 
ter would eafily diftinguilh two kinds of planets, four 
nearer him, viz. his fatellites, and two more remote, viz. 
the Sun and Saturn : the former however will fall vaftly 
fhort of the Sun in brightnefs, notwithftanding the great 
dilproportion in the diftances and apparent magnitude. 
Of the Satellites of Jupiter. 
The words moon and fatellite are fometimes ufed indif¬ 
ferently : thus we fay, either Jupiter’s moons, or Jupiter’s 
fatellites ; but ufually we diftinguilh, reftraining the term 
Moon to the Earth’s attendant, and applying the term fa¬ 
tellite to the moons more recently difeovered about JuptL 
ter, Saturn, and the Georgian planet. Thefe fatellites 
move round their primary planets, as their centres, by 
the fame laws as thofe primary ones do round their centre 
the Sun, viz. in fuch manner that, in the fatellites of the 
fame planet, the fquares of the periodic times are propor¬ 
tional to the cubes of their diftances from the primary 
planet. There are four of thefe fatellites, or fecondary 
planets, performing their revolutions about Jupiter, as that 
planet does about the Sun. Simon Marius, mathematician 
of the elector of Brandenburg, about the end of Novem¬ 
ber 1609, firft obferved three of them ; and in January 
1610, he found a fourth. In January 1610 Galileo alfo ob¬ 
ferved the fame in Italy, and in the fame year publilhed 
his obfervations. Thefe fatellites were alfo obferved in 
the fame month of January 1610, by Thomas Harriot, 
the celebrated author of a work upon algebra, and who 
made conftant obfervations of thefe fatellites, from that 
time till the 26th of February 1612; as appears by his cu¬ 
rious aftronomical papers, lately difeovered by Dr. Zach, at 
the feat of the earl of Egremont, near Petworth in Sulfex, 
Anthony. Maria Schyrlaeus di Rieta, a capuchin of Co¬ 
logne, imagined that, belides the four known fatellites of 
Jupiter, he had difeovered five more, on December 29, 
1642. But the obfervation being communicated to Gaf- 
fendus, who had obferved Jupiter on the fame day, he 
foon perceived that the monk had miftaken five fixed ftars, 
in the effufion of the water of Aquaries, marked in Tycho’s 
catalogue 24, 25, 26, 27, 28, for fatellites of Jupite;-. 
When Jupiter comes into a line between any of his fa¬ 
tellites and the Sun, the fatellite difappears, being then 
eclipfed, or involved in his fhadow. When the fatellite 
goes behind the body of Jupiter with refpect to an ob- 
ferver on the Earth, it is then faid to be occulted, being 
hid from our fight by his body, whether in his fhadow or 
not. And, when the fatellite comes into a polition between 
Jupiter and the Sun, it cafts a fhadow upon the face of 
that planet, which we fee as an obfeure round fpot. And, 
laftly, when the fatellite comes into a line between Jupiter 
and us, it is faid to tranfit the difk of the planet. It is 
not often, however, that a fatellite can be difeovered upon 
the dilk of Jupiter, even by the beft telefcopes, excepting 
at its firft entrance, when, by reafon of its being more di¬ 
rectly illuminated by the rays of the Sun than the planet 
itfelf, it appears like a lucid fpot upon it; fometimes, 
5 F however^ 
