OK, who was it who discovered the obliquity of the ecliptic? I should remember this from freshman astronomy, but I don't.
I'm pretty sure that it was Copernicus,
Actually Copernicus thought these were simple circles. Kepler got it right with eliptic orbits and also described the exact obliquity mathematically (based on his Tycho Brahe's years of meticulous observations.
The elliptical orbit was certainly Kepler, but I thought the fact that the plane of the orbit was not parallel to equator was known already. Copernicus
did know that the Sun was not at the center of the solar system; what took Tycho's observations and Kepler's mathematical models was the ellipticity.
I will try to get the full info next time I am near the library (my own history of science collection doesn't pin this down exactly). - DvF
