Our planet, Earth, can be approximated as a sphere of radius . Spheres are curved in three-dimensional space, but the paper and screens we read from are two-dimensional. So it is necessary to create a two-dimensional coordinate system that we can use to map out the planet. The most straightforward approach, if we are considering the world to be a sphere, would be to use spherical coordinates; is constant, so we can describe any point on Earth in terms of and . Or, more precisely, we describe points\