This universe theory is congruent with the geocentric model as opposed to the heliocentric model. However, such selection of "geocentric" or "heliocentric" frames is merely a matter of computation. Geodetic latitude and geocentric latitude represent similar quantities with different definitions. Geodetic GPS receivers produce directly three-dimensional coordinates in a geocentric coordinate frame. Astronomical clocks usually represent the solar system using the geocentric model. The orbits of planets in the once popular geocentric Ptolemaic system are epitrochoids. The geocentric and geodetic longitude have the same value. These geocentric ellipsoids are usually within of the geoid. Geocentric altitude is typically used in orbital mechanics. More technically, it is the mean semi-major axis of the geocentric lunar orbit. Their worldview was not exactly geocentric either. Ptolemy used and wrote about the geocentric system, drawing greatly on traditional Aristotelian physics. The following is a list of different geocentric orbit classifications. GPS navigation relies on such geocentric coordinates. ET's direct successor for measuring time on a geocentric basis was Terrestrial Dynamical Time (TDT).