Neither Mercury nor Venus has a moon, but Mars has two moons. This
statement, standing alone, might lead to the conclusion that, as far as
the advantages a satellite can afford to the inhabitants of its master
planet are concerned, the people of Mars are doubly fortunate. So they
would be, perhaps, if Mars's moons were bodies comparable in size with
our moon, but in fact they are hardly more than a pair of very
entertaining astronomical toys. The larger of the two, Phobos, is
believed to be about seven miles in diameter; the smaller, Deimos, only
five or six miles. Their dimensions thus resemble those of the more
minute of the asteroids, and the suggestion has even been made that they
may be captured asteroids which have fallen under the gravitational
control of Mars.
The diameters just mentioned are Professor Pickering's estimates, based
on the amount of light the little satellites reflect, for they are much
too small to present measurable disks. Deimos is 14,600 miles from the
center of Mars and 12,500 miles from its surface. Phobos is 5,800 miles
from the center of the planet and only 3,700 from the surface. Deimos
completes a revolution about the planet in thirty hours and eighteen
minutes, and Phobos in the astonishingly short period--although, of
course, it is in strict accord with the law of gravitation and in that
sense not astonishing--of seven hours and thirty-nine minutes.
Since Mars takes twenty-four hours and thirty-seven 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.
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