Thābit is believed to have been an astronomer of Caliph al-Mu'tadid. Thābit was able to use his mathematical work on the examination of Ptolemaic astronomy. The medieval astronomical theory of the trepidation of the equinoxes is often attributed to Thābit. But it had already been described by Theon of Alexandria in his comments of the Handy Tables of Ptolemy. According to Copernicus, Thābit determined the length of the sidereal year as 365 days, 6 hours, 9 minutes and 12 seconds (an error of 2 seconds). Copernicus based his claim on the Latin text attributed to Thābit. Thābit published his observations of the Sun. In regards to Ptolemy's Planetary Hypotheses, Thābit examined the problems of the motion of the Sun and Moon, and the theory of sundials. When looking at Ptolemy's Hypotheses, Thābit ibn Qurra found the Sidereal year which is when looking at the Earth and measuring it against the background of fixed stars, it will have a constant value.