On January 7, 1610, Italian mathematician and physicist Galileo Galilei turned his telescope toward Jupiter.
He saw three “stars” near it, “stars” that lay along its path of motion across the sky. He thought at first that they were ordinary stars, but being along that path seemed rather odd.
He observed Jupiter on the next day, and he noticed that those three “stars” had moved. Were they somehow connected to Jupiter?
He continued to observe them, and he discovered a fourth one. After a few more nights of watching and studying their motions, he became convinced that they were orbiting Jupiter.
One of his colleagues, Simon Marius, claimed to have observed them before Galileo did, and to have started recording his observations about when Galileo did. But he did not go into as much detail, and he did not publish as quickly as Galileo had published, so Galileo got the credit.
Galileo made several other important discoveries. He discovered that Aristotle had been wrong about falling objects by doing experiments on them, he developed some of the first telescopes, and he discovered the mountains of the Moon, the phases of Venus, sunspots, and numerous stars. He concluded that heliocentrism was likely true, and he tried to convince the Church not to condemn it as heretical, failing very famously.
More broadly, Galileo had discovered some strong evidence against a common belief back then, that the celestial regions were fundamentally different from the terrestrial regions. It became apparent that the Moon and the planets were Earthlike objects, even if different in detail, and that the rest of the Universe was not fundamentally different from the Earth.
- Galileo and the Mountains of the Moon
- Francesco Sizzi: Jupiter’s Moons Cannot Exist
- Jupiter’s Big Moons (Fast)
- Jupiter’s Big Moons (Slow)