150. Appearance.—The one remaining member of the inner group, Mars, has in recent years received more attention than any other planet, and the newspapers and magazines have announced marvelous things concerning it: that it is inhabited by a race of beings superior in intelligence to men; that the work of their hands may be seen upon the face of the planet; that we should endeavor to communicate with them, if indeed they are not already sending messages to us, etc.—all of which is certainly important, if true, but it rests upon a very slender foundation of evidence, a part of which we shall have to consider.
Beginning with facts of which there is no doubt, this ruddy-colored planet, which usually shines about as brightly as a star of the first magnitude, sometimes displays more than tenfold this brilliancy, surpassing every other planet save Venus and presenting at these times especially favorable opportunities for the study of its surface. The explanation of this increase of brilliancy is, of course, that the planet approaches unusually near to the earth, and we have already seen from a consideration of [Fig. 17] that this can only happen in the months of August and September. The last favorable epoch of this kind was in 1894. From [Fig. 17] the student should determine when the next one will come.
[Fig. 95] presents nine drawings of the planet made at one of the epochs of close approach to the earth, and shows that its face bears certain faint markings which, though inconspicuous, are fixed and permanent features of the planet. The dark triangular projection in the lower half of the second drawing was seen and sketched by Huyghens, 1659 A. D. In [Fig. 96] some of these markings are shown much more plainly, but [Fig. 95] gives a better idea of their usual appearance in the telescope.
151. Rotation.—It may be seen readily enough, from a comparison of the first two sketches of [Fig. 95], that the planet rotates about an axis, and from a more extensive study it is found to be very like the earth in this respect, turning once in 24h. 37m. around an axis tipped from being perpendicular to the plane of its orbit about a degree and a half more than is the earth's axis. Since it is this inclination of the axis which is the cause of changing seasons upon the earth, there must be similar changes, winter and summer, as well as day and night, upon Mars, only each season is longer there than here in the same proportion that its year is longer than ours—i e., nearly two to one. It is summer in the northern hemisphere of Mars whenever the sun, as seen from Mars, stands in that constellation which is nearest the point of the sky toward which the planet's axis points. But this axis points toward the constellation Cygnus, and Alpha Cygni is the bright star nearest the north pole of Mars. As Pisces is the zodiacal constellation nearest to Cygnus, it must be summer in the northern hemisphere of Mars when the sun is in Pisces, or, turning the proposition about, it must be summer in the southern hemisphere of Mars when the planet, as seen from the sun, lies in the direction of Pisces.
152. The polar caps.—One effect of the changing seasons upon Mars is shown in [Fig. 97], where we have a series of drawings of the region about its south pole made in 1894, on dates between May 21st and December 10th. Show from [Fig. 17] that during this time it was summer in the region here shown. Mars crossed the prime radius in 1894 on September 5th. The striking thing in these pictures is the white spot surrounding the pole, which shrinks in size from the beginning to near the end of the series, and then disappears altogether. The spot came back again a year later, and like a similar spot at the north pole of the planet it waxes in the winter and wanes during the summer of Mars in endless succession.