the planet’s axial rotation, so that it rises, like the other heavenly bodies, in the east; but, because its motion is so nearly equal, in angular velocity, to that of Mars’s rotation, it shifts very slowly through the sky toward the west, and for two or three successive days and nights it remains above the horizon, the sun overtaking and passing it again and again, while, in the meantime, its protean face swiftly changes from full circle to half-moon, from half-moon to crescent, from crescent back to half, and from half to full, and so on without ceasing.

And during this time Phobos is rushing through the sky in the opposite direction, as if in defiance of the fundamental law of celestial revolution, making a complete circuit three times every twenty-four hours, and changing the shape of its disk four times as rapidly as Deimos does!  Truly, if we were suddenly transported to Mars, we might well believe that we had arrived in the mother world of lunatics, and that its two moons were bewitched.  Yet it must not be supposed that all the peculiarities just mentioned would be clearly seen from the surface of Mars by eyes like ours.  The phases of Phobos would probably be discernible to the naked eye, but those of Deimos would require a telescope in order to be seen, for, notwithstanding their nearness to the planet, Mars’s moons are inconspicuous phenomena even to the Martians themselves.  Professor Young’s estimate is that Phobos may shed upon Mars one-sixtieth and Deimos one-twelve-hundredth as much reflected moonlight as our moon sends to the earth.  Accordingly, a “moonlit night” on Mars can have no such charm as we associate with the phrase.  But it is surely a tribute to the power and perfection of our telescopes that we have been able to discover the existence of objects so minute and inconspicuous, situated at a distance of many millions of miles, and half concealed by the glaring light of the planet close around which they revolve.

If Mars’s moons were as massive as our moon is they would raise tremendous tides upon Mars, and would affect the circulation of water in the canals, but, in fact, their tidal effects are even more insignificant than their light-giving powers.  But for astronomers on Mars they would be objects of absorbing interest.

Upon quitting Mars we pass to the second distinctive planetary group of the solar system, that of the asteroids.

CHAPTER V

THE ASTEROIDS, A FAMILY OF DWARF WORLDS

Beyond Mars, in the broad gap separating the terrestrial from the Jovian planets, are the asteroids, of which nearly five hundred have been discovered and designated by individual names or numbers.  But any statement concerning the known number of asteroids can remain valid for but a short time, because new ones are continually found, especially by the aid of photography.  Very few of the asteroids are of measurable size.  Among these are the four that were the first to be discovered—­Ceres, Pallas, Juno, and Vesta.  Their diameters, according to the measurements of Prof.  E.E.  Barnard, of the Yerkes Observatory, are as follows:  Ceres, 477 miles; Pallas, 304 miles; Juno, 120 miles; Vesta, 239 miles.