ASTRONOMY. 



[SiTUBH. 



The four satellite* of Jupiter are constantly cen in the 

 MUM direction, and almost in the same straight line ; aud 

 the motion* of the two inner ones round 

 th.-ir primary are Tery rapid. They almost 

 describe circles around the planet ; the orhiu 

 of the third and fourth satellites are, how- 

 ever, slightly eccentric. The following Uble 

 hows their mean dirt*""*'' from the centre 

 of the planet, expressed in the equatorial 

 diameter of Jupiter. The diagram (Fig. 69) 

 represent* the relative orbits of the four 

 satellites in exact proportion. 



i 



rings, of different form and brilliancy, and eight satellites, 

 with the varied bands which, as in the case of Jupiter, 



Fig. 70. 



Mean di-tanc. Period of revolution. 



1st satellite . . 3-03 . . 177 days. 

 2nd . . 4-81 . . 3-55 

 3rd . . 7-C8 . . 715 

 4th . . 13-50 . . 16-69 

 The laws of Kepler are as strictly fulfilled in the 

 passage of the satellites round their primary, as in the 

 case of the motions of the planets round the sun. 

 They are subject to eclipses and occultations, and 

 are sometimes seen projected on the limb in the 

 form of small white spots. As the cone of the 

 shadow is of considerable length, Jupiter being at 

 a great distance from the sun, and the satellites 

 comparatively near it, it follows that, at every 

 revolution, the first three satellites are immersed 

 in the shadow, and it is only the fourth which oc- 

 casionally escapes. The use made of the eclipses 

 of Jupiter's satellites, is represented by A, of the 

 engraving at the end of Chapter IV., page 940. The 

 centre globe being our earth, two lines are drawn 

 from places on its surface, forming an angle, and 

 meeting at Jupiter ; an observer at each place 

 marks the time at which the satellite enters 

 the cone of the shadow. Assuming the time at the 

 nearest line to be ten, and the farthest eight o'clock, the 

 two hours 30 show the one place to be distant 30 from 

 the other. These observations of the eclipses give the 

 readiest means of determining the longitude of any place 

 at which they are observed. This accurate observation 

 of the moment of disappearance depends partly on the 

 excellence of the telescope used, and partly on the keen- 

 ness of sight and experience of the observer. When the 

 satellites pass between the sun and Jupiter, they produce 

 solar eclipses ; aud the shadow of the satellite, in addition 

 to the satellite itself, may be seen slowly traversing thu 

 disc of the planet, in a powerful instrument. 



RM. The most wonderfully constituted body of 

 we solar system is Saturn. Surrounded by numerous 



cross the ball of tho planet, it forms the most curious 

 of telescopic objects. The ring was first detected by 

 Huygens, by means of his powerful telescopes, and 

 he was likewise tho first to perceive the brightest 

 of the satellites. He explained the various appear- 

 ances at different times by taking into account the 

 obliquity of the ring, and the parallelism which it 

 retains at all parts of its orbit ; which is, indeed, 

 similar in afl respects to the change of seasons on 

 the earth. This will easily be seen by the diagram 

 (Fig. 70), where S is the sun, and T the earth, and 

 the outer circle the extent of Satxirn's orbit. It 

 is plain, that to an observer at T, the upper side of 

 the ring, and the northern pole of Saturn, will be 

 visible, as at Fig. 71- At the opposite part of the 

 orbit of Saturn, the southern pole of the planet and 

 the under side of the ring is that which will be 

 illuminated by the sun, and scon by an observer 

 on the earth. As Saturn passes from the one to the 

 other of these points, the ring will become gradually 

 less open ; and at the points between the two it 

 will only be visible by the illuminated edge, as in 

 Figs. 72 and 73 ; whilst at other times, the unillii- 

 minod plane of the ring will be turned to an observer 

 at T, aud the outer edges of the ring will not be 

 visible, as at Fig. 70. This disappearance of tho 

 ring will take place twice during every revolution 

 of Saturn, and it may be compared to the position 

 of the earth's axis at the times of the equinoxes 

 in respect to the sun, both poles coming into sight, for 

 the ring of Saturn surrounds the equator of the plane. 



The ring of Saturn is seen, with an instrument of 

 moderate power, to be divided into two parts ; but when a 

 stronger telescope is made use of, under favourable cir- 

 cumstances, the outer portion is found to be divided into 

 two likewise ; thus making three in all, as seen in the 



Kg. 71. 



diagram, Fig. 71. Some observers have been able to 

 il.-irrt other divisions in the bright ring when it in must 

 open. The most remarkable appearance detected of late 

 years is, however, the dusky ring, situated between th<- 

 ball and the bright ring, and adjoining the latter. Tim 

 existence of this curious appendage is now placed beyond 

 doubt, by the testimony of different observers ; and it ap- 

 pears to be of a misty, semi-transparont nature, as the 

 hall of the planet can bo scon through it. From the 

 observations of tho elder Herschel, it would follow that 

 the glol>e of Saturn revolves on its axis once in lOh. 16m., 

 and that the ring makes a rotation in its plane once in 

 lOh. 32m. 15. Both move in the direction of the other 

 planetary bodies viz., from west to east. 



