JUPITER. 
another, but not always so; and their 
breadth is likewise variable, one belt hav- 
ing been observed to glow narrow, while 
another in its neighbourhood has increased 
in breadth, as if tlie one had flowed into 
the other, and in this case Dr. Long ob- 
serves, that a part of an oblique belt lay 
between them, as if to form a communica- 
tion for tliis purpose. The lime of their 
continuance is very uticertain, sometimes 
remaining unchanged for three months; 
at others, new belts have been formed in 
an hour or two. In some of these belts 
large black spots have appeared, which 
moved swiftly over the disk from east to 
west, and returned in a short time to tiie 
same place; from whence the rotation of 
this planet about its axis has been deter- 
mined. 
The figure of Jupiter is evidently an 
oblate spheroid, the longest diameter of 
his disk being to the shortest as 13 to 12. 
His rotation is from west to east, like that 
of the sun, and the plane of his equator is 
very nearly coincident with that of his 
orbit; so that there can scarcely be any 
difl^rence of seasons in that planet. His 
rotation has been observed to be some- 
what quicker in his aphelion than his perihe- 
lion. The axis of rotation is nearly per- 
pendicular to the plane of the ecliptic, and 
the planet makes one revolution in about 
9 h. 55' and 37". The changes in the 
appearances of these spots, and the differ- 
ence in the time of their rotation, nrake 
it probable that they do not adhere to 
Jupiter, but are clouds transported by the 
wind, with different velocities, in an atmos- 
phere subject to violent agitations. 
Four little stars are observed around 
Jupiter, which constantly accompany him. 
Then' relative situation is continually chang- 
ing. They oscillate on both sides of tlie 
planet, and their relative rank is determin- 
ed by the length of these oscillations. That 
one in which the oscillation is shortest is 
called the first satellite, and so on. These 
satellites are analogous to our moon. See 
Astronomy. They are all supposed to 
move in ellipses; though the excentri- 
cities of all of them are too small to be mea- 
sured, excepting that of the fourth; and 
even this amounts to no more than 0.007 
of its mean distance from the primar)'. 
The orbits of these planets were thought 
by Galileo to be in the same plaiie with 
that of their primary : but M. Cassini has 
found that then- orbits make a small angle 
with it; and as he did not find any differ- 
VOL. IV. 
ence in the place of their nodes, he con- 
cluded that they were all in the same place, 
and that their ascending nodes were in the 
middle of Aquarius. After observing them 
for more than thirty-six years, he found 
their greatest latitude, or deviation from 
the plane of Jupiter’s orbit, to be 2° 55'. 
The first of these satellites revolves at the 
distance of 5.697 of Jupiter’s- semi-diame- 
ters, or 1' 5l", as measured by proper in- 
slruments ; its periodical time is T*. 18'*. 
27' 34". The next satellite revolves at the 
distance of 9.017 semi-diameters, or 2' 56 ', 
in S*!. IS**. 13' 43" ; the third at the distance 
of 14,384 semi-diametei's, or 4' 42", in 7'‘. 
o'*. 42' 36''; and the fourth at the distance 
of 25.266, or 8' 16", in 16‘‘. 16'*. 32' 09". 
Since the time of Cassini it has been found 
that the nodes of Jupiter’s satellites are 
not in the same place; and from the dif- 
ferent points of view in which we have an 
opportunity of observing them from the 
earth, we see them sometimes apparently 
moving in straight lines, and at other times 
in elliptic curves. All of tliem, by reason 
of their immense distance, seem to keep 
near their primary, and their apparent 
motion is a kind of oscillation like that of 
a pendulum ; going alternately from their 
greatest distance on one side to the great- 
est distance on the other, sometimes in a 
straight line, and sometimes in an elliptic 
curve. 
When a satellite is in its superior semi- 
circle, or that half of its orbit which is 
more distant from the earth than Jupiter 
is, its motion appears to us direct, accord- 
ing to the order of the signs ; but in its infe- 
rior semi-circle, when it is nearer to us f han 
Jupiter, its motion appears retrogade ; and 
both these motions seem quicker the nearer 
the satellites are to the centre of the pri- 
mary, slower the more distant they are, 
and at the greatest distance of all they ap- 
pear for a short time to be stationary. 
From this account of tlie system of Jupiter 
apd his satellites, it is evident that occulta- 
tions of them must frequently happen by 
their going behind their primary, or by 
coming in betwixt us and it. The former 
takes place when they proceed towards 
the middle of their upper semi-circle ; the 
latter, when they pass through the same 
part of their inferior semi-circle. Occiilta- 
tions of the former kind happen to the 
first and second satellites ; at every revolu- 
tion, the third very rarely escapes an occul- 
tation ; but the fourth more frequently, by 
reason of its greater distance. 
C 
