I have been looking for the answer to this question
everywhere and cannot find it. Who discovered that the earth was on
23.5 degree tilt and when was it accepted?
I don't know who is credited with determining that the
Earth is tilted, but scientific observers/astronomers
surely knew it by the time of the invention of the
telescope. The effects of the tilt were apparent even
to the ancients, by 2000 BC, as they were building
pyramids and astrological devices (stone circles, etc.)
to allow them to track the seasons. Eventually Milutin
Milankovitch used the tilt and the variation of it with
time, plus the Earth's orbit around the Sun to
help explain long term climate changes.
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Update: June 2012