Double-polar coordinates can be converted to Cartesian coordinates by the trigonometric sine function and the Pythagorean Theorem: \(x=r\sin(\alpha)\) \(y=r\sin(\epsilon)\) \(z=\sqrt{r^2-x^2-y^2}\) Other coordinate systems include the spherical and polar coordinate system ( Fig. 4 ). The polar coordinate system is useful in describing rotations of the eye, while the spherical system is