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another orb which would move the apogee with the observed motion.

The other phenomenon that concerned Ibn al-Shatir was the change in the apparent diameter of the Sun. According to Ptolemy, the diameter could be regarded as a constant, and he gave the value 0; 31, 20° though, in his solar theory, the ratio of maximum to minimum distance is about 0.92. From the observations available to him, Ibn al-Shatir came to the erroneous conclusion that the diameter at apogee was 0; 29, 5° and at perigee it was 0; 36, 55°, with a mean value of 0; 32, 32°. This implied thatthe ratio of maximum to minimum apparent diameter and, hence, the ratio of the solar distance at apogee to that at perigee, was about0.788, significantly different from the value used by Ptolemy.

Ibn al-Shatir's solution is illustrated in Figure 4.6. The Earth is E and the point D moves around a circle centred on E such that the line ED always points toward the mean sun S. This circle is called the 'parecliptic' and is taken to have a radius of 60 units. The parecliptic carries a smaller circle - the deferent -centred at D, that rotates in the opposite direction at the same rate, so that the line DB connecting the centre of the deferent to a point on its circumference

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