The orbit of Mars is very eccentric, more eccentric, in fact, than any other planet in the solar system with the exception of Mercury. At its nearest approach to the earth's orbit it is only 35,000.000 miles away, while at its furthermost point it is distant 62,000.000 miles. The two planets, however, when at different points in their orbits, may be over 200,000,000 miles apart. This eccentricity in the orbit of our neighbor Mars causes it to differ in brightness as we see it shining as a star, for, although it is always large and beautiful, at its nearest approach to the earth it shines with more than tenfold the brilliancy of a first magnitude star. At such a time its fiery light attracts the attention of everyone. Such extreme increase in light can only occur in the months of August and September and then only about every fifteen years. This was a much heralded astronomical event of the year 1924, when the two planets were closer together than any other time during more than a hundred years. However, Mars was low in the sky and not in such a good position for observation as it will 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: 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 permanent markings on its surface. The first thing one notices through a telescope is the ruddy color of its surface and the dark belt across the equatorial regions. The ruddy color is presumed to be the characteristic color of its soil, five-eighths of which is in a desert condition; there are also regions of other colors which are subject to change, and a white cap marking either pole.