THE PLANETS. 403 



the sequence of their discovery, their volumes compared either 

 with each other, or with their distances from the sun ; accord- 

 ing to their relative densities, masses, periods of rotation, 

 degrees of eccentricity, the inclinations of their axes, and charac- 

 teristic differences within and beyond the zone of the small 

 planets. In the comparative contemplation of these subjects, 

 it is consistent with the nature of this work, to bestow espe- 

 cial attention upon the selection of the numerical relations, 

 which, at the period in which these pages appear, are con- 

 sidered to be the most accurate, i. e. the results of the most 

 recent and reliable investigations. 



a. PRINCIPAL PLANETS. 



1. Number and epoch of discovery. Of the seven cosmical 

 bodies which, from the most remote antiquity, have been 

 distinguished by their constantly varying relative position 

 towards each other from those which apparently maintain the 

 same positions and distances, the scintillating stars of the 

 region of fixed stars [orbis inerrans] there are only five 

 which appear star-like, "quinque stellce errantes ;" they are 

 Mercury, Venus, Mars, Jupiter, and Saturn. The Sun and 

 the Moon remained almost separated from the others, since 

 they form large discs, and also on account of the greater 

 importance attached to them in accordance with religious 

 myths. 2 Thus, according to Diodorus (ii. 30), the Chaldeans 

 were acquainted with only five planets. Plato also says 

 distinctly in the Timaus, where he only once mentions the 

 planets, " Round the Earth, fixed in the centre of the Cosmos, 



2 Gesenius, in the Hallischcn Litter atur-Zeitung, 1822, 

 No?. 101 and 102 (Supplement, pp. 801-812). Among the 

 Chaldeans, the Sun and Moon were held to be the two princi- 

 pal deities ; the five planets merely represented genii. 



VOL. iv. . I 



