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.
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