THE PLANETS. 



13^ 



the third satellite of Jupiter. The densest of this group of 

 satellites, the second, is even denser than Jupiter himself, 

 while the third and largest appears to be oi equal density 

 with the primary. The masses also do not increase in at all 

 the same ratio as the distances. ' If the planets have been 

 formed from revolving rings, then the greater or less dense 

 aggregation round a nucleus must have been caused by pe- 

 cuUar causes, which may, perhaps, always remain unknown 

 to us. 



The orbits of the secondary planets which belong to the 

 same group have very different degrees of eccentricity. In 

 the Jovial system, the orbits of the first and second satellites 

 are nearly circular, while the eccentricities of those of the 

 third and fourth satellites amount to 0*0013 and 00072. In 

 the Saturnian system, the orbit of the satellite nearest to the 

 primary (Mimas) is considerably more eccentric than the orb- 

 its of Enceladus and Titan, the largest and first discovered, 

 whose orbit was so accurately determined by Bessel. The 

 eccentricity of the orbit of the sixth satellite of Saturn is only 

 0-02922. According to all these data, which are among those 

 that may be relied upon, Mimas only is more eccentric than 

 the Earth's Moon (0-05484) ; the latter possesses the pecul- 

 iarity that its orbit round the Earth has a greater eccentric- 

 ity, in comparison with that of its primary, than any other 

 satellite. Mimas revolves round Saturn in an orbit whose 

 eccentricity is 0-068, while that of the orbit of its primary is 

 0-056 ; but the orbit of our Moon has an eccentricity of 0-054. 

 while the eccentricity of that of the Earth is only 0-016. With 

 regard to the distances of the satellites from their primaries, 

 compare Cosmos, vol. i., p. 94-98. The distance of the sat- 

 ellite nearest to Saturn (Mimas) is now no longer taken as 

 80,088 geographical miles, but as 102,400 ; whence its dis- 

 tance from the ring, this being calculated as 24,188 miles 

 broad, and at a distance of 18,376 miles from the surface of 

 the planet, will be 28,000 miles. ^ Remarkable anomalies, 

 together with a certain correspondence, are also presented in 

 the position of the orbits of the satellites in the Jovial sys- 

 tem, in which very nearly all the satellites move in the plane 

 of the equator of their primary. In the group of Saturnian 

 satellites, seven of them revolve almost in the plane of the 

 ring, while the outermost (the eighth, Japetus) is inclined to- 

 ward their plane 12° 14'. 



* In the earlier data (Cosmos, vol. i., p . 97) the equatorial rliametei 

 was taken as a basis. 



