Tycho Brahe suggested a geocentric system which explained phases of Venus before they were even discovered.

AFAIK the first serious "proof" (as far as we can talk about proving that the Sun is the center of everything) was the observation of stellar parallax some century later. In Galileo's time parallax was too small to detect and it was an open question whether stars are too far to observe it (postulated by Copernicus) or Earth is actually immobile (postulated by Tycho Brahe et al).