PORTION OF THE COSMOS. THE PLANETS. 369 



Mars. 



The diameter of this planet is 892 German, or 3568 

 English geographical miles, (being only 0'519 of the Earth's 

 diameter, notwithstanding its considerably greater solar 

 distance). The excentricity of its orbit is 0*0932168; 

 being, next to that of Mercury, the greatest among the 

 old planets. This circumstance, together with its proxi- 

 mity to the Earth, rendered it the best adapted to lead to 

 Kepler's great discovery of the elliptic orbits of the pla- 

 nets. The rotation of Mars ( 597 ) is, according to Madler 

 and Willielm Beer, 24 hours, 37 minutes, and 23 seconds. 

 The sidereal period of revolution round the Sun is 1 year, 

 321 days, 17 hours, 30 minutes, 41 seconds. The inclina- 

 tion of the orbit of Mars to the terrestrial Equator is 24 

 44' 24" ; the mass of the planet ati ^ 6337 ; and its density, 

 in comparison with that of the Earth, 0'958. As the great 

 approximation of Encke's comet was made use of for in- 

 vestigating the mass of Mercury, so the mass of Mars will 

 some day be ascertained with greater accuracy by the per- 

 turbations which it may cause in the movements of De 

 Vico's comet. 



The compression of the planet Mars at its poles, which, 

 singularly enough, was always doubted by the great astro- 

 nomer of Koiiigsberg, was first recognised by William 

 Herschel in 1784. In respect to the amount of the ellip- 

 ticity, however, uncertainty long prevailed. William Her- 

 schel gave it as T V; according to Arago's more exact 

 measurement ( 598 ) with a prismatic telescope of Rochon, 

 it would be much less : in 1824 he found it as 189 : 194, 

 or -3-^-5- ; and, by a later measurement in 1847, it was found 

 3 V Arago, however, is inclined to believe that the com- 



