variability is ascertained.

The period of the mean precession (i.e. the time it takes for the mean north pole to complete 1 revolution about the pole of the ecliptic) was calculated by Copernicus as 25 816 Egyptian years (an Egyptian year being 365 days) which is very close to the modern value. The values computed from Copernicus' elaborate theory give excellent agreement with the observational data that he was attempting to reproduce.

Finally, Copernicus provided his theory for the shape of the orbit of the Earth. This can be thought of as an extension of the solar theory of Hipparchus which, if transferred to a heliocentric viewpoint, consists simply of the Earth moving round the Sun on an eccentric circle. Copernicus believed that the orbit of the Earth was subject to two further inequalities. The first of these concerned the eccentricity of the orbit of the Earth, which from historical measurements he thought was variable, and he made the highly questionable assumption that it oscillated with the same period as the obliquity. Second, there was the motion of the aphelion of the Earth. As we have already seen (see p. 97) al-Zarqalidiscovered that the apogee of the Sun had a slow steady motion with respect to the fixed stars, but Copernicus misunderstood al-Zarqal?s

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