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and others to explain the retrograde mo-
tions of the planets. We, who know that
the earth revolves round the sun once in a
year, can understand that the apparent mo-
tion of a planet is only its motion relative
to the earth. If, then, we suppose the earth
fixed and the sun to revolve round it once a
year, and the planets each in its own period,
it is only necessary to impose upon each of
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