Four hundred years ago at the University of Padua in Italy, a professor of mathematics named Galileo Galilei learned of a remarkable invention. Dutch lensmaker Hans Lippershey had taken some lenses and created a device with them that makes distant objects look nearer, what we now call a telescope. Galileo decided to make one for himself, designing improved models and showing an 8x one to city officials of Venice. They were impressed enough to give him a big raise. He got up to 20x magnification, and in November that year, he looked at the Moon with it.
Near the dividing line between the light and dark areas was some light and dark spots that came and went as that dividing line moved. After observing the Moon for several days, Galileo concluded that he was seeing mountains and he estimated their heights from their illumination and shadowing. There went the old doctrine of heavenly perfection and spotlessness, with the Moon being only a little bit contaminated.
On January 7 the next year, he turned his telescope toward Jupiter and he saw three small “stars” in a line with it. He observed that planet for the next several days, watching them move and discovering a fourth one a week later. He concluded that they move around Jupiter the way that the Moon moves around the Earth.
He also discovered that Venus shows a full set of Moon-like phases, that Saturn has two mysterious objects near it, that the Sun has spots, and that the Milky Way is composed of an enormous quantity of faint stars.
Galileo’s discoveries provoked responses from his colleagues ranging from getting their own telescopes and observing with them to refusing to look through his telescope to extremely preposterous ones. A certain Francesco Sizzi claimed that Jupiter’s moons could not exist because the seven traditional planets correspond to the seven openings of the human head, the seven metals, the seven days of the week, etc. But for anyone willing to experiment with a telescope, it was hard to deny what Galileo had seen.
Galileo noted how well his discoveries supported heliocentrism. Venus’s orbit is clearly centered on the Sun, and Jupiter’s moons are clearly not moving around the Earth. Galileo famously got in trouble with the Church about heliocentrism, despite his valiant efforts to argue that the Bible tells us how to go to heaven, not how the heavens go. The Church wanted it presented as only an unproved theory, and though Galileo agreed, he wrote a book where the geocentrist came off as foolish. This pissed off Church officials enough to make him recant heliocentrism and to ban his books.
Galileo’s successors designed fancier and fancier telescopes, telescopes that work by reflection as well as by refraction, and so forth. Nowadays, for a few hundred dollars or its monetary equivalent, you can buy a telescope that will outperform Galileo’s telescope and see what he had seen — and more. Or use a friend’s telescope. So enjoy seeing what he had seen and thinking about what a revolution his discoveries had been.