I am wondering about the origins of the Unit Circle. Of course it is part of trigonometry, which goes back many centuries. But since it uses Cartesian coordinates, it should be after Descartes.
So, that's my question, when did the unit circle come into general use, and is it's definition attributable to a specific mathematician?
Edit: I found this question, where someone says Euler played a big part in this. Is this correct? Could someone elaborate?