Here’s a big one:
What were those problems? A big one was the size of stars in the Copernican universe. Copernicus proposed that certain oddities observed in the movements of planets through the constellations were due to the fact that Earth itself was moving. Stars show no such oddities, so Copernicus had to theorise that, rather than being just beyond the planets as astronomers had traditionally supposed, stars were so incredibly distant that Earth’s motion was insignificant by comparison. But seen from Earth, stars appear as dots of certain sizes or magnitudes. The only way stars could be so incredibly distant and have such sizes was if they were all incredibly huge, every last one dwarfing the Sun. Tycho Brahe, the most prominent astronomer of the era and a favourite of the Establishment, thought this was absurd, while Peter Crüger, a leading Polish mathematician, wondered how the Copernican system could ever survive in the face of the star-size problem.Christopher Graney, “Opposition to Galileo was scientific, not just religious” at Aeon
And more. We know so much more about the universe now that this doesn’t seem like a problem. But in those days, it had to be.
Pop science writing typically misleads us by portraying the conflict as if the rightness of the Copernican universe were self-evident. For sure not at the time.