Who discovered the unit circle?

     There is actually no one person, that took credit for discovering the unit circle, but there are aspects of the unit circle that other people previously discovered.
     The unit circle lies on the Cartesian coordinate system , which Rene Descartes discovered in 1637. It is a way to find points in a plane using two points called x-coordinate and y-coordinate.
The Cartesian Coordinate System is in the Euclidean plane, which was discovered by a Greek Mathemetician named Euclid.
Radians are used to represent angle measures in the unit circle. Roger Cotes, an English Mathematician created this concept of using Radian measures to represent angle measures in 1714.