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times of opposition. Their diameters are
estimated at six or seven miles. It was soon
found that the first, Deimos, completes its
orbit in 30h. 18m. But the other, Phobos,
at first was a puzzle, owing to its incred-
ible velocity being unsuspected. Later it
was found that the period of revolution was
only 7h. 39m. 22s. Since the Martian day
is twenty-four and a half hours, this leads
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