As regards the tilt of their orbits, some are actually as much as
34 degrees inclined to the ecliptic, so that in fact they are
seen from the Earth among our polar constellations.
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From all this you see that Mars occupies a rather hot comer in
the solar system. Is it not possible that more than once in the
remote past Mars may have encountered one of these wanderers? If
he came within a certain distance of the small body his great
mass would sway it from its orbit, and under certain conditions
he would pick up a satellite in this manner. That his present
satellites were actually so acquired is the suggestion of Newton,
of Yale College.
Mars' satellites are indeed suspiciously and most abnormally
small. I have not time to prove this to you by comparison with
the other worlds of the solar system. In fact, they were not
discovered till 1877—although they were predicted in a most
curious manner, with the most uncannily accurate details, by
Swift.
One of these bodies is about 36 miles in diameter. This is
Phobos. Phobos is only 3.700 miles from the surface of Mars. The
other is smaller and further off. He is named Deimos, and his
diameter is only 10 miles. He is 12,500 miles from Mars' surface.
With the exception of Phobos the next smallest satellite known in
the solar system is one of Saturn's—Hyperion; almost 800 miles in
diameter. The inner one goes all round Mars in 7½ hours. This is
Phobos' month. Mars turns on his axis in 24 hours and 40 minutes,
so that people in Mars would see the rise of Phobos twice in the
course of a day and night; lie would apparently cross the sky
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going against the other satellite; that is, he would move
apparently from west to east.
We may at least assume as probable that other satellites have
been gathered by Mars in the past from the army of asteroids.
Some of the satellites so picked up would be direct: that is,
would move round the planet in the direction of his axial
rotation. Others, on the chances, would be retrograde: that is,
would move against his axial rotation. They would describe orbits
making the same various angles with the ecliptic as do the
asteroids; and we may be sure they would be of the same varying
dimensions.
We go on to inquire what would be the consequence to Mars of such
captures.