Fig. 580.—Venus, near inferior conjunction.
Whether Venus has a constitution similar to our globe is of course doubtful. The matter is less dense than the earth, and there is an atmosphere half as dense again as ours. Spots have been noticed crossing the planet, which may have been vapours or clouds, and the rotation of Venus on its axis was calculated from these spots as being 23h 21m 22s. The seasons in Venus must be very different from ours, as her inclination is greater than our earth, and as the sun is so much nearer to her than to us her tropical and polar regions are close, and a vertical sun is scarcely enjoyed by two places for three successive days, and she may have two winters and summers, two springs and autumns!
Mars.
Having already considered the earth, we pass on from Venus to Mars. The orbit of the latter planet is exterior to the earth’s, as is proved by his never appearing “horned,” nor ever passing across the sun’s disc. Therefore no “transits” of Mars can take place as transits of Venus and Mercury.
Yet Mars is most favourably situated for astronomical observation by us, because it turns its full disc to us. Venus is nearer to us than Mars—but, as we have explained, when she comes nearest to us she is quite invisible. Astronomers have been enabled to ascertain a good deal concerning the planet of war—“the red planet Mars.”
Mars has been considered very like the earth. We perceive seas and continents, and the shape of Mars is like the earth. But our globe is larger than Mars, which is much less dense, so the force of gravitation is less also. Mars moves upon his axis in about twenty-four hours and a half, and takes rather more than 686 days to revolve round the sun. (See page 489.) Thus its days are a little longer than, and its years twice as long as our days and years. When in “opposition,” or on the opposite side of us from the sun, Mars is at his brightest. This happened in September 1877. He will come close again to us in 1892.
Fig. 581.—Mars seen from the earth.
All planets are wanderers, but of all the wanderers Mars has the most eccentric orbit. He curls about, so to speak, in loops and curves in a very irregular manner, and therefore his distance from the earth varies very considerably; and this eccentric behaviour of the warlike planet must have, as we believe it did, puzzled the ancients very much. But—and here reason came to human aid—this very fact, this great eccentricity of the planetary motions, caused Copernicus to investigate the subject with great attention, and he at length explained the true reason of these irregular orbits from the hypothesis that it was around the sun, and not around the earth that the planets moved in regular orbits.
It is quite ascertained that Mars is very like our earth in miniature. We annex a diagram of the planet, and when it is examined with a good telescope the seas and continents can be quite distinctly perceived. At the poles there appears to be a white or snowy region at varying periods, which would lead us to the conclusion that the atmospheric changes and the seasons are similar to our own; and as the inclination of the planet is nearly the same as the earth, this supposition may be accepted as a fact.