The shape of the earth is approximated by an ellipsoid. Given a point P on the surface
of the earth. Let the geocentric latitude be the angle northwards between the line from P
to the center of the earth and plane of the equator. Denote the geocentric latitude by t.
Denote the geodetic latitude, the angle between the normal to the surface and the
equatorial plane, by 
. I will always use radians.