I was reading how Galileo's discovery of bodies orbiting Jupiter was the catalyst for the realisation that the "fact" that the Sun and moon seem to be orbiting the Earth, doesn't necessarily have to imply that we're the center of the Universe. Galileo observed this between January 7 and March 2, 1610, and published this is in his essay "The Starry Messenger" that same month. However, in that very same essay he casually mentions Venus' and Mercury's orbits of the Sun.
So which discovery was it then? Which was the first proof of a body orbiting something else? The moons of Jupiter orbiting Jupiter or Venus and Mercury orbiting the Sun? And why are we all attributing the start of the change from a geocentric to a heliocentric view to his discovery of Jupiter's moons, rather than his(?) discovery of Venus and Mercury's orbits around the Sun?
Excerpts from "The Starry Messenger" where he casually mentions Venus and Mercury's orbit of the sun:
> But that 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 Erratic Stars, neither known nor observed by any one of the astronomers before my time, which have their revolutions 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 depan from it beyond certain limits.
(Note: "Four Erratic Stars" refers to the four bodies he saw orbiting Jupiter, and "a certain bright star" to Jupiter)
> 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, observations of whose changes of position made with more exactness on succeeding nights the following account will supply
Thank you.
edit: I can't seem to figure out how to put those excerpts into quotes, so I've put them in italics for now.
Here is how Mario Biagioli talks about this argument:
In the Nuncius Galileo compared the Medicean Stars to Venus and Mercury in the sense that they were “wandering stars” that did not orbit the Earth (SN, p. 36). Although, according to Ptolemy and Aristotle, Mercury and Venus orbited the Earth, there was a plausible minority position among astronomers (and a nonheretical one) according to which Mercury and Venus went around the Sun. This explained the fact that their elongation was always limited. Tycho’s model incorporated that position. Galileo, in sum, tried to “normalize” the Medicean Stars by arguing that they were like Venus and Mercury. One could argue that this was also a way to point to the fact that more wandering stars were found that did not orbit the Earth—an oblique pro-Copernican claim.
Which seems right to me — it's kind of Copernican, kind of Tychonian, but in any case, not officially against geocentrism and thus not heretical. The Tychonic model also has Venus and Mercury orbiting the Sun rather closely, but the Sun itself orbits a stationary Earth.
I think though, to your broader question, the idea that the moons of Jupiter made Galileo a Copernican is just not true. Galileo was already a Copernican well before this. He had started using Copernicanism in this astronomical theorizing in 1595, and in 1597 he wrote several letters to others indicating his preference for Copernicanism over the alternatives. Which is to say, as with many people in the history of science, Galileo came to accept the theory before there were strong evidence for it, and interpreted the evidence he did find in that light. There is no sin in this; it is quite common, although it contradicts the more idealistic and naive models of how scientists function. With Copernicanism you can more or less say that anyone who was a Copernican in the time of Galileo was operating under a similar structure, because the decisive evidence that could distinguish between a Copernican and Tychonic (geocentric) worldview did not exist until the 19th century. Some of the Copernicans of that time (e.g. Kepler) were very explicit about the fact that their justifications for believing it were beyond that of evidence and observation (for Kepler it was due to his unusual religious beliefs, which amounted to a form of sun-worship).
Which is only to say: Galileo didn't discover the moons of Jupiter and say, "oh, it is Copernicanism after all!" He was already a Copernican, and he reported the moons of Jupiter with this in mind. In Siderius Nuncius, he does not actually endorse any particular model of astronomy, but it is clear from the passages you have marked that he is nodding towrads either a Copernican or a Tychonic one (and not a Ptolemaic/Aristotelian model).
The Biagioli quote comes from Mario Biagioli, Galileo's Instruments of Credit: Telescopes, Images, Secrecy (University of Chicago Press, 2006), 143, fn. 22.