> Aristarchus had come up with a heliocentric theory way back around 250 BC.
He notably did convincing calculations showing that the sun should be larger than Earth. However, this was not convincing enough for his opponents who argued notably that usually fire hardly remains in a fixed position. Also, if Earth was moving, it was hard to understand why there was no observed parallax on the stars at night. Actually, the first apparent proof that Earth was moving came after inventing instruments precise enough to observe parallax on far enough stars, in 1727, from James Bradley. It actually had different results from expectation, due to the finite speed of light [1].
Just after Aristarchus came also Seleucus of Seleucia [2]. He supposedly had a theoretical argument for heliocentrism, but it was lost.
I have recently been listening to the "The Complete History of Science" podcast [1]. I have not gotten to the Copernicus portion yet, but I really enjoyed the ancient Greek astronomy episodes (including the episode on Aristarchus).
They believed that the stars and Earth moved relative to each other, as far as I can tell. Why does it matter if they thought the Earth was moving or if they thought the stars were moving?
The night sky obviously rotates, but that the earth translates relative to the stars is not so obvious. This was especially a problem because it was generally thought the stars were much closer than they actually are, because stars do have an apparent diameter to the naked eye (and through a telescope) which we now know to be an optical illusion (Airy Disk), and if you extrapolate this like with the planets they are either not that much further away or ludicrously big, like many solar systems, and parallax measurements were precise enough to at least put a high bound on their distance if the earth moved with respect to the stars, and gigantic stars wasn't a popular option.
He notably did convincing calculations showing that the sun should be larger than Earth. However, this was not convincing enough for his opponents who argued notably that usually fire hardly remains in a fixed position. Also, if Earth was moving, it was hard to understand why there was no observed parallax on the stars at night. Actually, the first apparent proof that Earth was moving came after inventing instruments precise enough to observe parallax on far enough stars, in 1727, from James Bradley. It actually had different results from expectation, due to the finite speed of light [1].
Just after Aristarchus came also Seleucus of Seleucia [2]. He supposedly had a theoretical argument for heliocentrism, but it was lost.
[0]: https://en.wikipedia.org/wiki/On_the_Sizes_and_Distances_(Ar...
[1]: https://en.wikipedia.org/wiki/James_Bradley
[2]: https://en.wikipedia.org/wiki/Seleucus_of_Seleucia