Page 260 - 89 history of astronomy_opt

Then he proved that the effect ought to be
a rotation of the earth’s axis over a con-
ical surface in space, exactly as the axis
of a top describes a cone, if the top has
a sharp point, and is set spinning and dis-
placed from the vertical. He actually cal-
culated the amount; and so he explained
the cause of the precession of the equinoxes
discovered by Hipparchus about 150 B.C.
260