SECT. IX. KOTATION OF THE SU^ T . 65 



SECTION IX. 



Rotation of the Sun and Planets Saturn's Rings Periods of the Rota- 

 tion of the Moon and other Satellites equal to the Periods of their Revo- 

 lutions Form of Lunar Spheroid Libration, Aspect, and Constitution 

 of the Moon Rotation of Jupiter's Satellites. 



THE oblate form of several of the planets indicates rotatory 

 motion. This has been confirmed in most cases by tracing spots 

 on their surface, by which their poles and times of rotation have 

 been determined. The rotation of Mercury is unknown, on 

 account of his proximity to the sun ; that of the new planets has 

 not yet been ascertained. The sun revolves in twenty-five days 

 and ten hours about an axis which is directed towards a point half- 

 way between the pole-star and a of Lyra, the plane of rotation 

 being inclined by 7 30', or a little more than seven degrees, to the 

 plane of the ecliptic : it may therefore be concluded that the sun's 

 mass is a spheroid, flattened at the poles. From the rotation of 

 the sun, there was every reason to believe that he has a pro- 

 gressive motion in space, a circumstance which is confirmed by 

 observation. But, in consequence of the reaction of the planets, 

 he describes a small irregular % orbit about the centre of gravity 

 of the system, never deviating from his position by more than 

 twice his own diameter, or a little more than seven times the 

 distance of the moon from the. earth. The sun and all his 

 attendants rotate from west to east, on axes that remain nearly 

 parallel to themselves (N. 140) in every point of their orbit, 

 and with angular velocities that are sensibly uniform (N. 141). 

 Although the uniformity in the direction of their rotation is a 

 circumstance hitherto unaccounted for in the economy of nature, 

 yet, from the design and adaptation of every other part to the 

 perfection of the whole, a coincidence so remarkable cannot be 

 accidental. And, as the revolutions of the planets and satellites 

 are also from west to east, it is evident that both must have 

 arisen from the primitive cause which determined the planetary 

 motions.* Indeed, La Place has computed the probability to be 



* The satellites of the two great planets on the farthest verge of the 

 solar system form a singular exception to this law. 



