60 ASTRONOMICAL TABLES. SKCT. IX 



Jupiter. In the present state of astronomy, the masses 

 and elements of the orbits are pretty well known^ so 

 that the tables only require to be corrected from time 

 to time, as observations become more accurate. Those 

 containing the motions of Jupiter, Saturn, and Uranus, 

 have already been twice constructed within the last thirty 

 years. The tables of Jupiter and Saturn agree almost 

 perfectly with modern observation ; those of Uranus, 

 however, are already defective, probably because the 

 discovery of that planet in 1781, is too recent to admit 

 of much precision in the determination of its motions, 

 or that possibly it may be subject to disturbances from 

 some unseen planet revolving about the sun beyond the 

 present boundaries of our system. If, after a lapse of 

 years, the tables formed from a combination of numer- 

 ous observations should be still inadequate to represent 

 the motions of Uranus, the discrepancies may reveal 

 the existence, nay even the mass and orbit of a body 

 placed forever beyond the sphere of vision. 



The tables of Mars, Venus, Mercury, and even those 

 of the sun, have been greatly improved, and still occupy 

 the attention of Professor Airy and other distinguished 

 astronomers. We are chiefly indebted to the German 

 astronomers for tables of the four new planets, which 

 are astonishingly perfect, considering that these bodies 

 have not been discovered more than forty years, and a 

 much longer time is requisite to develop their inequal- 

 ities. 



SECTION IX. 



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

 of the Moon and other Satellites equal to the Periods of their Revolu- 

 tions Form of Lunar Spheroid Libratjon, Aspect, and Constitution of 

 the Moon Rotation of Jupiter's Satellites. 



THE oblate form of several ot 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 prox- 

 imity to the sun ; that of the new planets has not yet 



