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to have a bar with one end centred at the
earth. If at the other end of the bar one
end of a shorter bar is pivotted, having the
planet at its other end, then the planet is
given an annual motion in the secondary
circle (the epicycle), whose centre revolves
round the earth on the primary circle (the
deferent ), at a uniform rate round the ex-
centric. Ptolemy supposed the centres of
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