en minutes for one
rotation on its axis, it is evident that Phobos goes round the planet
three times in the course of a single Martian day and night, rising,
contrary to the general motion of the heavens, in the west, running in a
few hours through all the phases that our moon exhibits in the course of
a month, and setting, where the sun and all the stars rise, in the east.
Deimos, on the other hand, has a period of revolution five or six hours
longer than that of 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