120 COSMOS. 



ets. The differences of density which are presented by Mars, 

 Venus, the Earth, and even Mercury, are very slight ; almost 

 equally similar among each other, but from 4 to 7 times less 

 dense than the former group, are the planets more distant 

 from the Sun — Jupiter, Neptune, Uranus, and Saturn. The 

 density of the Sun (0-252, if the Earth is taken as 1*000 ; 

 therefore, in reference to water, 1'37) is but little more than 

 the densities of Jupiter and Neptune. Consequently, the 

 planets and the Sun* must be arranged, according to their 

 increasing density, in the following order : 



Saturn, Uranus, Neptune, Jupiter, Sun, Venus, Mars, Earth, 

 Mercury. 



Although, upon the whole, the densest planets are nearer to 

 the Sun, still, when they are considered individually, their 

 density is by no means proportional to the distances, as New- 

 ton was inclined to assume.! 



7. Periods of Sidereal Revolution and Axial Rotation. 

 — We shall confine ourselves here to giving the sidereal, or 

 true periods of revolution of the planets in reference to the 

 fixed stars, or a fixed point of the heavens. During such a 

 revolution, a planet passes through exactly 360 degrees in its 

 course round the Sun. The sidereal revolutions of the plan- 

 ets must be clearly distinguished from the tropical and synodic, 

 the former of which refer to the return to the spring equinox, 

 the latter to the difference of time between two consecutive 

 conjunctions or oppositions. 



* The Sun (which Kepler considered to be magnetic, probably from 

 enthusiastic admiration for the divina inventa of his justly famous co- 

 temporary, William Gilbert, and whose rotation in the same direction 

 as the planets he maintained long before the Sun-spots were discovered) 

 Kepler declares, in his Comment, de motibus Stella Martis (cap. 23), and 

 in Astronomies pars Optica (cap. 6), to be " the densest of all cosmical 

 bodies, because it moves all the others which belong to his system." 



t Newton, De Mundi Systemate, in Opusculis, torn, ii., p. 17: ''Cor- 

 pora Veneris et Mercurii majore Solis calore magis concocta et coagu- 

 lata sunt. Planetae ulteriores, defectu caloris, carent substantiis illis 

 metallicis et mineris ponderosis quibus Terra referta est. Densiora cor- 

 pora quae Soli propiora : ea ratione constabit optime pondera Planeta- 

 rum omnium esse inter se ut vires." " The bodies of Venus and Mer- 

 cury are more ripened and condensed on account of the greater heat 

 of the Sun. The more remote planets, by want of heat, are deficient 

 in those metallic substances and weighty minerals with which the Earth 

 abounds. Bodies are denser in proportion to their nearness to the Sun; 

 from which reason it will easily appear that the weight c fall planets is 

 in proportion to their forces." 



