As on example, to find longitude and latitude on a sphere from stereographic projection (given as Cartesian coordinates) you need both: [ x, y ] ↦ [ atan2(y, x

As on example, to find longitude and latitude on a sphere from stereographic projection (given as Cartesian coordinates) you need both: [ x, y ] ↦ [ atan2(y, x), π/2 − 2 atan(√(x² + y²)) ] But atan per se comes up all over the place if you start doing much work with angle measures.