He was the tutor of Alexander of Aphrodisias, and wrote a work "On Revolving Spheres". On 4th September 164 AD, he watched an 'annular' solar eclipse. He noticed that, unlike a total solar eclipse, the apparent size of the moon was smaller than that of the sun. This meant the distances between the sun, moon and earth were not the same at different eclipses. For this reason, sun and moon could not be on a circular movement around the same centre.