As an astronomer, he discovered, among other things, that Jupiter has moons, that the Milky Way is composed of stars, that the moon has mountains, and that Venus has moonlike phases.
Astronoomina avastas ta lisaks muule, et Jupiteril on kuud, et Linnutee koosneb tähtedest, et Kuul on mäed ning et Veenusel on kuutaolised faasid.