The Sun Sets in the West and Rises in the East
The Complete History of Science
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The Origins of Ptolemy's Theories
Ptolemy had estimated the rate of procession between his own time and Hipparchus at roughly one degree per century. But when the Arab astronomers looked, they found the change was only about one degree every sixty-five years. By the mid-ninth century then, there were two clear discrepancies in Ptolemy's measurements that needed to be accounted for. The most successful of these attempts was the theory of trepidation attributed to Thabbat Ibn Khura. He attempted to find a model which simultaneously accounted for the change in the obliquity of the ecliptic, and the change inthe rate of procession. His attempts unfortunately were hammed strong by
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