Four points of light beside Jupiter proved not everything in the sky orbits Earth

In January 1610, Galileo pointed a homemade telescope at Jupiter and noticed four small 'stars' that changed position every night, always lined up with the planet. He realized they were moons orbiting Jupiter itself, not Earth — direct proof that at least one other body in the heavens had satellites of its own. It became key evidence against the Earth-centered universe.

— Galileo Galilei, Sidereus Nuncius (The Starry Messenger) — published 1610