THEORETICAL ASTRONOMY. 117 
17’ 35”, 18’ 0”, and 8’ 54”. The true diameters are for I., 2,120; for II., 
1,880 ; for pe 3,120; and for 1V., 2,640 English geographical miles ; their 
densities are 1, 2, .3,, and } that ef the earth. | 
34. The blaliet Saturn, on his 30 years’ journey around the sun, is accom- 
panied by seven moons, revolving round him from west to east. Huyghens 
discovered one of these moons, siantiolye the sixth, March 25,1655. Cassini 
found the seventh and most distant on October 25, 1671 the fifth on Decem- 
ber 13,1672; as also in March, 1684, the third and fourth. One hundred years 
after, August 28 and Sept. 17, 1789, Herschel] discovered the two satellites 
nearest the planet. The orbits of the six inner moons (pi. 10, fig. 7) are 
nearly circular, and lie almost entirely in the plane of Saturn’s ring; the 
orbit of the seventh, however, lies nearly in the plane of the ecliptic. Their 
periods of revolution are 0 days, 22 hours, 38 minutes; 1 day, 8 hours, 53 
minutes ; 1 day, 21 hours, 18 minutes ; 2 days, 17 hours, 45 minutes; 4 days, 
12 hours, 25 minutes ; 15 days, 22 hours, 41 minutes; and 79 days, 7 hours, 
55 minutes. Their mean distances from Saturn amount, in geographical 
miles, to 76,680, 99,640, 163.880, 211,680, 295,480, 642,840, 2,098,744. 
With regard to the true magnitudes, masses, and densities of the satellites 
of Saturn, nothing satisfactory is known, as these moons are among the 
smallest and most remote objects of the heavens. Schroter estimated the 
true diameter of the fifth and sixth satellites at 1,040 and 2,720 geographical 
-miles. It is only since 1830 that the sixth moon has received a more 
accurate determination by Bessel. 
The first and second moons had only been seen ee their discoverer 
Herschel, until in 18836 Camont found the second, and in June 27, 1838, 
the astronomers at Rome were enabled to observe the first? The seventh 
or outermost moon revolves about Saturn at the great distance of 2,098,744 
geographical miles, and has the remarkable peculiarity of almost entirely 
vanishing when to the east of Saturn, and of shining brightest in its western 
elongation. This is probably produced by. the fact of its completing a 
rotation about its axis in the time that it is accomplishing a revolution about 
Saturn, presenting the same side to the earth when it comes into the same 
position with respect to its primary—the one side reflecting the sun’s 
light much more completely than the other, consequently the equality of the 
time of rotation and revolution in the secondary planets appears to apply to 
these bodies also. The moons of Saturn are sometimes eclipsed, and some- 
times produce eclipses of the sun to inhabitants of Saturn ; nevertheless, both 
kinds of eclipses, which always follow closely the time of the disappearance 
of the ring, occur more rarely than in the case of Jupiter, a consequence 
of the great inclination of their orbits to that of Saturn (pl. 10, fig. 7). 
Yet eclipses of Saturn’s moons often occur among themselves, and are also 
caused by the ring. 
35. Uranus, on his 84 years’ journey about the sun, is accompanied by six 
moons (fig. 8). Herschel, on January 11, 1787, discovered the second and 
fourth; on January 18, 1790, the first; on February 9, 1790, the fifth; on 
February 28, 1'794, the sixth ; and on March 26, 1794, the third satellite of 
Uranus. These moons are at the following successive distances from their 
Ld 
