CHAPTER IX.

MARS a. ♂

Period, &c.-Phases.-Apparent motions.-Its brilliancy.—Telescopic appearance. Its ruddy hue.-Schiaparelli's Canals."-General statement of the physical details of Mars.-Map of Mars on Mercator's projection.—Polar snow.- -Axial rotation.-The seasons of Mars.-Its atmosphere.-The Satellites of Mars.-Ancient observation of Mars.--Tables of Mars.

MARS is the first planet exterior to the Earth in the order

of distance from the Sun, and, as we shall presently see, bears a closer analogy to it than do any of the other planets.

Mars revolves round the Sun in 686d 23h 30m 418, at a mean distance of 141,536,000 miles, which an orbital eccentricity of 0.093 may augment to 154,714,000 miles, or diminish to 128,358,000 miles. The apparent diameter of Mars varies between 4.1′′ in conjunction and 30.4" in opposition; and owing to the great eccentricity of the orbit of Mars its apparent diameter as seen from the Earth will vary much at different oppositions. The diameter at mean distance of the planet from the Earth being 7.28′′ (Le Verrier), the real diameter is nearly 5000 miles. Very varying results have been arrived at as to the compression of Mars. Sir W. Herschel gave it at; Schröter contradicted this, and asserted that it must be less than; Bessel merely decided that it was too

a Observers interested in Mars should consult a valuable memoir entitled Areographie presented to the Académie Royale de Belgique in June 1874 by F. Terby of Louvain. It is the most

exhaustive account of the planet which has ever appeared. A fine series of lithographic views by N. E. Green will be found in Mem. R.A.S., vol. xliv. p. 123, 1879.