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supposed to revolve in the deferent round
the earth in their own periodic times, and
in their epicycles once in a year.
It did not occur to Ptolemy to place
the centres of the epicycles of Mercury and
Venus at the sun, and to extend the same
system to the major planets. Something of
this sort had been proposed by the Egyp-
tians (we are told by Cicero and others),
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