A good description of how to make transformations between the different
coordinate systems can be found in a paper by M. A. Hapgood, "Space physics
coordinate transformations: A user guide", in Planetary and Space Science,
Vol. 40, No. 5, pp. 711-717, 1992.
Geocentric inertial reference frame using the Julian Epoch of Jan. 1, 2000.
At the Julian Epoch,
X = First point of Aries (Vernal Equinox, i.e. from Earth to the Sun in the first day of Spring)