THE MAJOR PLANETS. 



253 



The orbits of the six inner satellites are nearly in the plane of the ring, but 

 that of the most remote one is inclined to it at the rather large angle of 30. 



Owing to the great obliquity of the orbits of the satellites to that of Saturn, 

 they are seldom eclipsed. The frequency of the eclipses of the satellites of 

 Jupiter, is a consequence of the fact that their orbits are nearly in the plane of 

 that of the planet. 



The most remote of Saturn's moons (commonly called the fifth satellite) 

 exhibits variations of brilliancy which have given ground for the conjecture 

 that those moons, like our own and those of Jupiter, revolve on their axes in 

 the time they take to revolve in their orbits. 



The two innermost satellites were the latest discovered, and are by far the 

 most difficult to be seen. It is only by means of telescopes of the most power- 

 ful kind, and under circumstances most favorable to observation, that they can 

 be detected at all. Those who have been so fortunate as to possess instru- 

 ments capable of observing them, say that at the equinoxes of Saturn, when 

 his ring becomes invisible, they have been seen threading like beads the al- 

 most infinitely thin filament of light to which the ring is then reduced, and for 

 a short time moving off it at either end, speedily to return, and hastening again 

 to their habitual concealment. 



OF HERSCHEL, OR URANUS. 



The planet of the solar system which is the most remote from the sun, and 

 which, there are strong reasons for believing to be the extreme limit of the 

 system, is called Uranus, and sometimes, from its distinguished discover- 

 er, Herschel. This body is a globe 35,000 miles in diameter, the bulk of 

 which is about eighty times that of the earth ; and it revolves at a distance 

 from the sun of eighteen hundred millions of miles ; being double the dis- 

 tance of Saturn. The great distance of this object from the earth and the 

 consequent minuteness of its appearance, has rendered our knowledge of its 

 physical condition much less distinct and satisfactory than those of the nearer 

 planets. 



It has been hitherto unascertained whether it has a diurnal rotation ; but 

 analogy favors the conjecture that it revolves rapidly upon its axis like the 

 cognate planets, Jupiter and Saturn. The disk has not been seen with suffi- 

 cient distinctness to detect upon it those indications which would decide the 

 question, whether it is invested with an atmosphere. 



The period for this planet going round the sun is eighty-four terrestrial years, 

 and as the date of its discovery was 1781, it has not yet made a complete rev- 

 olution since astronomical observation was first directed to it. It is a striking 

 example of the power of science, that we are nevertheless as certainly assured 

 of its periodical path round the sun, as if it had been observed for a long suc- 

 cession of its periods like other planets. 



Being nearly twenty times farther from the sun than the earth, the diameter 

 of the sun will appear to it proportionally less ; and as the sun's apparent diameter 

 at the earth is thirty minutes, it will subtend at Herschel at an angle of only a 

 minute and a half. "We subjoin here a diagram in which, if we suppose the 

 larger circle E, to represent the appearance of the sun as seen from the earth ; 

 the smaller one H, will represent its appearance as seen from Herschel. 



As the intensity of solar light diminishes in the same proportion as the su- 

 perficial magnitude of the sun's disk diminishes, it will follow that the bright- 

 ness of day at the planet Herschel must be between three and four hundred 

 times less than at the earth ! We might be led, however, from such a numer- i 

 ical estimate to form a very incorrect estimate of what the solar light under ] 



