Thābit was also an author and wrote De Anno Solis. This book contained and recorded facts about the evolution in astronomy in the ninth century. Thābit mentioned in the book that Ptolemy and Hipparchus believed that the movement of stars is consistent with the movement commonly found in planets. What Thābit believed is that this idea can be broadened to include the Sun and Moon. With that in mind, he also thought that the solar year should be calculated by looking at the Sun's return to a given star.