RichD wrote:
When and how did astronomers determine that the earth's axis
tilts from the ecliptic, and its value?


I guess the answer depends on who you ask.

http://search.yahoo.com/search;_ylt=...8&fr=yfp-t-701


Just like Galileo is often credited for claiming the earth went around the
sun.
His research was based on Copernicus's work, and who knows where Copernicus
got the idea.