A spherical coordinate system uses three numbers to identify a point in space.: Usually, two angles, and a distance from the origin of the coordinate system. If the point lies on the sphere, only the two angles are needed, because the distance from the origin is known.
The two angular numbers are related to lines of longitude and latitude on Earth. Earth's longitude is the same as the variable phi, which can be written as either or The latitude resembles the variable (theta), except that some authors define to be at the north pole instead of at the equator.