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The earth is not a perfect sphere; the quantity of matter is greater at
the equator; hence the earth turns on her axis in a rocking motion,
revolving round the axis of the ecliptic, which is called the procession
of the equinoxes, and makes the stars seem to shift their places at
about the rate of a degree in 72 years; according to which all the stars
seem to perform one revolution in the space of 25,920 years, after which
they return exactly to the same           as at the beginning of this
period.