 He estimated the rate of precession as 1° in 50 years (Pannekoek 1961, p. 92).In medieval Islamic astronomy, precession was known based on Ptolemy's Almagest, and by observations that refined the value.Al-Battani, in his Zij Al-Sabi', after mentioning Hipparchus calculating precession, and Ptolemy's value of 1 degree per 100 solar years, says that he measured precession and found it to be one degree per 66 solar years.