JUPITER. 



swiftness till he comes in opposition. The 

 motion then becomes gradually slower, 

 and becomes direct when the planet ad- 

 vances within 115 of the sun. The du- 

 ration of the retrograde motion is about 

 121 days, and the arch of retrogradation 

 described is about 10. But there is a 

 considerable difference both in the amount 

 and in the duration of this retrograde 

 motion. 



Jupiter has the same general appear- 

 ance with Mars, only that the belts on his 

 surface are much larger and more perma- 

 nent They are said to have been first 

 discovered by Fontana and two other 

 Italians ; but Cassini was the first who 

 gave a good account of them. Their 

 number is very variable, as sometimes 

 only one, and at others no fewer than 

 eight, may be perceived. They are gene- 

 rally parallel to one another, but not al- 

 ways so ; and their breadth is likewise 

 variable, one belt having been observed 

 to grow narrow, while another in its neigh- 

 bourhood has increased in breadth, as if 

 the one had flowed into the other, and in 

 this case Dr. Long observes, that a part 

 of an oblique belt lay between them, as if 

 to form a communication for this pur- 

 pose. The time of their continuance is 

 very uncertain, sometimes remaining un- 

 changed 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 swift- 

 ly over the disk from east to west, and 

 returned in a short time to the same place ; 

 from whence the rotation of this planet 

 about its axis has been determined. 



The figure ot Jupiter is evidently an 

 oblate spheroid, the longest diameter of 

 his disk being to the shortest as 1-3 to 12. 

 His rotation is from west to east, like that 

 ot the sun, and the plane of his equator is 

 very nearly coincident with that of his 

 orbit ; so that there can scarcely be any 

 difference of seasons in that planet. His 

 rotation has been observed to be some- 

 what quicker in his aphelion than his pe- 

 rihelion. The axis of rotation is nearly 

 perpendicular 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 

 difference in the time of their rotation, 

 make it probable that they do not adhere 

 to Jupiter, but are clouds transported by 

 the wind, with different velocities, in 

 an atmosphere subject to violent agita- 

 tions. 



Four little stars are observed around 

 Jupiter, which constantly accompany him. 



Their relative situation is continually 

 chunging. They oscillate on both sides 

 of the planet, and their relative rank is 

 determined by the length of these oscilla- 

 tions. That one in which he oscillation 

 is shortest is called the firs satellite, and 

 so on. These satellites are analogous to 

 our moon. See ASTRONOMY. They are 

 all supposed to move in ellipses ; though 

 the eccentricities of all of them are too 

 small to be measured, excepting that of 

 the fourth ; and even this amounts to no 

 more than 0.007 of its mean distance from 

 the primary. 



The orbits of these planets were thought 

 by Galileo to be in the same plane with 

 that of their primary : but M. Cassini has 

 found that their orbits make a small angle 

 with it ; and as he did not find any differ- 

 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 ot these satellites 

 revolves at the distance of 5.697 of Ju- 

 piter's semi-diameters, or 1' 51", as mea- 

 sured by proper instruments ; its periodi- 

 cal time is K 18^. 27' 34". The next 

 satellite revolves at the distance of 9.017 

 semi-diameters, or 2' 56", in 3*. 13 h . 13 ; 

 43" ; the third at the distance of 14.384 

 semi-diameters, or 4' 42", in 7 d . 3 h . 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. A.H of them, by 

 reason of their immense distance, seem to 

 keep near their primary, and their appa- 

 rent motion is a kind of oscillation, like that 

 of a pendulum; going alternately from 

 their greatest distance on one side to the 

 greatest 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 

 interior semi-circle, when it is nearer to 

 us than Jupiter, its motion appears retro- 

 grade ; and both these motions seem 

 quicker the nearer the satellites are to 



