Were the moons of Jupiter that Galileo discovered with his telescope evidence in favor of Copernicus’s sun-centered (heliocentric) model of the universe? You sure do hear that they were. Consider this, from Smithsonian magazine:
But some of [Galileo’s] observations, especially the changing phases of Venus and the presence of moons around other planets, lent support to Copernicus’ heliocentric theory, and that made Galileo’s work potentially heretical.
(Galileo only discovered moons around one planet, Jupiter; the business about “heretical” is, of course, a dubious statement.)
Or this, from National Geographic:
Galileo’s discovery [of the moons] provided evidence for the Copernican understanding of the universe. This was the idea that everything in existence did not, indeed, move around Earth.
Or this, from Wikipedia:
Galileo’s discovery…. of celestial bodies orbiting something other than Earth dealt a blow to the then-accepted Ptolemaic world system, which held that Earth was at the center of the universe and all other celestial bodies revolved around it.
And indeed, Galileo spoke of the Jovian moons in pro-Copernican terms in his 1610 book Sidereal Messenger, in which he announced his discovery of the moons:
[S]ince they are sometimes behind, sometimes before Jupiter, at like distances, and withdraw from this planet towards the east and towards the west only within very narrow limits of divergence… it can be a matter of doubt to no one that they perform their revolutions about this planet…. Moreover, they revolve in unequal circles…. [and] the revolutions of the satellites which describe the smallest circles round Jupiter are the most rapid, for the satellites nearest to Jupiter are often to be seen in the east, when the day before they have appeared in the west, and contrariwise. Also the satellite moving in the greatest orbit seems to me, after carefully weighing the occasions of its returning to positions previously noticed, to have a periodic time of half a month.
Besides, we have a notable and splendid argument to remove the scruples of those who can tolerate the revolution of the planets round the Sun in the Copernican system, yet are so disturbed by the motion of one Moon about the Earth, while both accomplish an orbit of a year’s length about the Sun, that they consider that this theory of the constitution of the universe must be upset as impossible; for now we have not one planet only revolving about another, while both traverse a vast orbit about the Sun, but our sense of sight presents to us four satellites circling about Jupiter, like the Moon about the Earth, while the whole system travels over a mighty orbit about the Sun in the space of twelve years.
No wonder that if you ask an AI like the Bing AI about the history of astronomy, it tells you about Galileo and about Jupiter’s moons being evidence for, or even proof of, the Copernican system.
But there is a little more to this story. The Galileo quotation above (from The Sidereal messenger of Galileo Galilei, E.S. Carlos translation, London, 1880) is found toward the end of the Sidereal Messenger. Toward the very beginning, he states that the discovery he has made —
which will excite the greatest astonishment by far, and which indeed especially moved me to call the attention of all astronomers and philosophers, is this, namely, that I have discovered four planets, neither known nor observed by any one of the astronomers before my time, which have their orbits round a certain bright star, one of those previously known, like Venus and Mercury round the Sun, and are sometimes in front of it, sometimes behind it, though they never depart from it beyond certain limits.
Note: they have their orbits around Jupiter (the “bright star”) “like Venus and Mercury round the Sun”. And toward the middle of the book, Galileo says that,
I therefore concluded, and decided unhesitatingly, that there are three stars in the heavens moving about Jupiter, as Venus and Mercury round the Sun; which at length was established as clear as daylight by numerous other subsequent observations. These observations also established that there are not only three, but four, erratic sidereal bodies performing their revolutions round Jupiter….
Once again: “as Venus and Mercury round the Sun”.
Now, we cannot directly see Venus and Mercury circling the Sun like we can see the Jovian moons circling Jupiter. The Sun’s brilliance prevents that. But astronomers have long known that they do. It just requires plotting out their positions to see it. The two videos below show both the motions of Venus and Mercury around the Sun as seen from Earth, and the motions of the Jovian moons around Jupiter as seen from Earth.
(These videos are simulations made using the Celestia planetarium “app”.) You can see here why Galileo compared the moons to Venus and Mercury.
There were various explanations for why Venus and Mercury hang around the Sun the way they do. In the model of the universe developed by the ancient astronomer Ptolemy, Venus and Mercury both travelled on epicycles that remained always in front of the Sun. Ptolemy’s model was the dominant one, but there were alternative models available in which Venus and Mercury circled the Sun directly. All these models were Earth-centered.
The moons did not somehow show for the first time that celestial bodies could orbit something other than Earth. They did not deal a blow to the Ptolemaic world system, at least not for anyone who understood the motions of the planets as seen from Earth. The Jovian moon motions echoed those of Venus and Mercury.
Had you been around in 1610, and had you been a follower of Ptolemy and thought Venus and Mercury were on epicycles in front of the Sun, then you likely would have assumed that the Jovian moons were on epicycles in front of Jupiter. Then, when Galileo later announced seeing the changing phases of Venus, you would have had to switch to one of those alternative models where Venus and Mercury circled the Sun directly. If, on the other hand, you were a person who “thought outside the box” and you already held to an alternative Earth-centered model, then you probably assumed that the moons were circling Jupiter directly — and felt very smug when Galileo announced the Venus phases.
All this is in black and white in the Sidereal Messenger, and has been for over four centuries. Yet you will read over and over that the moons of Jupiter were evidence in favor of Copernicus’s heliocentric model of the universe. And now you can hear that from AIs, too.