Solar theory

After the mathematical preliminaries, Ptolemy discusses the motion of the Sun, and here he uses exactly the same model as that developed by Hipparchus. Ptolemy made some observations of the dates of the equinoxes so as to see whether Hipparchus' value for the length of the tropical year was still correct after a 300-year period, and he concluded that it was and that it has the value ascribed to it by Hipparchus, i.e. 365 days 5 h 55 min 12 s- about 6j minutes too long.

Ptolemy also used his observations of the equinoxes to recompute the length of the seasons and again found agreement with Hipparchus' values of 941 days for the length of the spring season and 92 2 days for the length of summer. He thus obtained the same numerical parameters for the eccentric model of the Sun, i.e. 65° 30' for the longitude of the solar apogee as measured from the vernal equinox, and 1 /24 for the eccentricity of the orbit. The fact that Ptolemy arrived at the same result for the solar apogee as Hipparchus had done

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