SATURN URANUS NEPTUNE.] 



ASTRONOMY. 



949 



The system of satellites which surround Saturn and 



his ring will be seen from the accompanying diagram 



Fig. 72. Fig. 73. 



(Fig. 74). Of 

 Huygens, and 



these the brightest is that discovered by 

 resembles a star of the ninth magnitude. 



Fig. 74. 



It is the sixth in the order of distance, counting from the 

 planet, and its orbit is the best determined of any of 

 them. The third, fourth, fifth, and eighth, discovered by 

 Dominique Cassini, are likewise comparatively bright ; 

 but the first and second, discovered by Herschel, and 

 the seventh, discovered by M. Sassel in 1848, are the 

 faintest of telescopic objects, and require both great 

 light and high power to be seen at all. The orbit of the 

 eighth satellite is considerably inclined to the equator of 

 the planet ; those of the other seven lie nearly in the same 

 plane as the ring. The mean distances, expressed in 

 equatorial radii of the planet, and the periods of their 

 revolutions, are seen from the following table : 



1st Satellite 



2nd 



3rd 



4th 



5th 



6th 



Tth 



8th 



Mean distance. 



. 3-35 . 



. 4-30 . 



. 5-28 . 



. 6-82 . 



9-52 . 



. 22-08 . 



. 2778 . 



. 64-36 . 



Time of Revolutions. 



. . 0-94 



. . 1-37 



. . 1-89 



. . 2-74 



. . 4-52 



. . 1594 



. . 22-50 



. . 79-33 



URANUS AND NEPTUNE. Of the telescopic appearance 

 of the two exterior planets, Uranus and Neptune, very 

 little can be said. The former appears in the telescope 

 a a star of the sixth magnitude, of a planetary aspect, 

 and with the appearance of a well-defined disc. The 



latter is of the same faint blue colour, but the disc is 

 more difficult to be perceived ; but, if compared with a 

 neighbouring star of the same magnitud 

 the difference in their definition is more 

 apparent. Herschel considered that he 

 was able to detect a slight ellipticity in the 

 figure of Uranus, from which it appeared 

 that its axis is very little inclined to the 

 plane of the elliptic, and that consequently 

 the sun is hid for many years from its 

 poles. 



The satellite system of Uranus (accord- 

 ing to Herschel) is represented in Fig. 75 ; 

 but only four of those satellites, two of 

 which do not agree in their periods with 

 that set down by Herschel, have been 

 perceived by M. Sassel ; and he seems 

 inclined to believe that only that number 

 exists. The second and fourth have fre- 

 quently been seen, and their orbits are pretty well deter- 

 mined by the observations of Sir J. Herschel and M. 

 Lament, of Munich. The orbits of those satellites are 

 very greatly inclined to the plane of the elliptic, the 

 angle between the two being nearly 80". The periods of 

 revolutions and mean distances of the several satellites 

 are seen from the following table : 



Fig. 75. 



lat Satellite 



2nd 



3rd 



4th 



5th 



6th 



Mean Distances- 



13-32 

 17-02 

 19-85 

 22-75 

 45-51 

 91-01 



Time of revolutions. 



5-89 



8-71 

 10-96 

 13-46 

 38-07 

 107 -C9 



76, on the f ^ 



;hat of the | T | 



of Jupiter, I / 



lus. It will \ / 



The only satellite to Neptune which has yet been per- 

 Fie 76. ceived is that discovered by Fiir. 77. 



M. Sassel, and it is repre- /- -v, 



sented in Fig. 76, on the * 



same scale as that 



satellite systems 



Saturn, and Uranus, n wm \ / 



be seen that it differs but ^~ 



slightly from that of the moon to the earth (Fig. 77) in 

 the dimensions of its orbit. The period of revolution 'is 

 5 '87 days, and its orbit is inclined at an angle of 35 on 

 the elliptic. It is excessively minute, and has hitherto, 

 been seen only by Messrs. Sassel and Bond. 



Having thus described the leading phenomena pre- 

 sented in the planetary system, we shall next proceed to, 

 consider the constellations, and the fixed stars generally! 



