He modified Eudoxus’ scheme of Earth-centred spheres, adding extra ones for the Sun, Moon, and some of the planets, bringing the total to 34. Callippus’ model was further refined by Aristotle. Callippus calculated accurate lengths for the seasons, as measured between solstice and equinox, and introduced the Callippic cycle, a period of 76 years after which lunar phases repeat on the same calendar dates; it contains 940 synodic months. The first Callippic cycle began on the June solstice of 330 bc.