 Although Al-Zaqali's work was based on the belief that the earth was at the center of the universe, many of his primary concepts still apply to the modern world of astronomy.Al-Zarqali designed his equatorium using the Ptolomaic astronomical model, which held that the earth was at the center of the universe with the stars fixed to a large sphere surrounding it.Ptolemy theorized that all of the stars moved around the earth every single day, and the motion of the sun, moon, and planets surrounded it.