The elements of astronomy; a textbook . s eastward, and retro-grades during 70. 329. Magnitude, Mass, Etc. — The apparent diameter of theplanet ranges from at conjunction to at a favorableopposition. Its real diameter is approximately 4300 miles, withan error of perhaps 50 miles one way or the other. This makesits surface about two-sevenths, and its volume one-seventh ofthe earths. Its mass is a little less than i of the earths mass. Thismakes its density and superficial gravity ; a bodywhich here weighs 100 pounds would have a weight of only38 pounds on the surface of Mars.
The elements of astronomy; a textbook . s eastward, and retro-grades during 70. 329. Magnitude, Mass, Etc. — The apparent diameter of theplanet ranges from at conjunction to at a favorableopposition. Its real diameter is approximately 4300 miles, withan error of perhaps 50 miles one way or the other. This makesits surface about two-sevenths, and its volume one-seventh ofthe earths. Its mass is a little less than i of the earths mass. Thismakes its density and superficial gravity ; a bodywhich here weighs 100 pounds would have a weight of only38 pounds on the surface of Mars. 330. General Telescopic Aspect, Phases, Albedo, Atmosphere,Etc. — When the planet is nearest the earth, it is more favor-ably situated1 for telescopic observation than any otherheavenly body,— the moon alone excepted. It then shows aruddy disc which, with a power of 75, is as large as the moon. 1 Venus at times comes nearer; but when nearest she is visible only bydaylight, and the hemisphere presented to the earth is mostly § 331] TELESCOPIC ASPECT. 231 Since its orbit is outside the earths, it never exhibits the crescent phases like Mercury and Venus; but at quadrature it appears distinctly gibbous, as in Fig. 82, about like the moon three days from the full. Like Mercury, Venus, and the moon, BS its disc is brighter at the limb (, at the 1|B circular edge) than at the centre; but at BjJL^zlj^B the terminator or boundary between day and night upon the planets surface, there is a slight shading which, taken in connection with certain other phenomena, indicates the presence of an atmosphere. Mars at Quadrature. This atmosphere, however, contrary toopinions formerly held, is probably much less dense than thatof the earth, the low density being indicated by the infre-quency of clouds and of other atmospheric phenomena famil-iar to us upon the earth, to say nothing of the fact that sincethe planets superficial gravity is less than § the force ofgravity on the ear
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