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.
Starp viņa astronomiskajiem atklājumiem ir Jupitera pavadoņi, Mēness kalni, Venēras fāzes, kā arī fakts, ka Piena Ceļš sastāv no zvaigznēm.