be two years from that date when at a higher altitude, even though it will be at a greater distance from the earth. During shorter periods at intervals of two years and two months, the earth passes between Mars and the sun; Mars then shines in our night-sky as an especially red and lovely star with a well-illumined face just right for observation.
Mars is a much smaller globe than either the earth or Venus, but it is larger than the planet Mercury._-_figure_151.jpg)
MARS, SHOWING SURFACE MARKINGS AND POLAR CAPS.
Photographs by the Mount Wilson Observatory. Its diameter of 4230 miles is only a little more than half that of the earth, although its period of rotation is almost the same, the earth gaining by two-thirds of an hour. Its day therefore is only about 40 minutes longer than ours. The seasons of Mars are twice as long as ours, and its year equals 687 of our days, or 669 of its own.
The atmosphere of Mars is very rare and thin, more rare and thin than that surrounding the peak of Mount Everest, our highest mountain. Being less than one-fourth as dense as that found on the earth, one may see through it easily and discern the perma-
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