Geographic coordinates:

This is a planetwide Earth-fixed coordinate system, shown in Fig. . The $z$-axis is defined to be parallel to the Earth's axis, in the direction of the Earth's north pole. The $x$-axis is defined to be parallel to the direction perpendicular from the Earth's rotation axis to a reference point in Greenwich, UK (the prime meridian. Note that we adopt a longitude convention that is consistent with the Astronomical Almanac, but opposite to that in [4], in that our geographic longitudes increase eastward (counterclockwise) like the rest of our longitudinal coordinates.

The terms ``latitude'' and ``longitude'' without qualification normally refer to geographic latitude and longitude. However, we emphasize once again that geographic latitude and longitude as defined above refer to directions in space, not to locations on the Earth's surface. This can lead to some confusion. The geodetic latitude and longitude of a point on the Earth's surface are the latitude and longitude of its vertical direction; this is the standard meaning used by cartographers, and relates directly to the horizon-based coordinate system above. However, one can also define a geocentric latitude and longitude for a point on the surface, which are the latitude and longitude of the direction from the geometric centre of the Earth through that point. These angles are not necessarily the same, due to the Earth's ellipticity, as shown in Fig.  in TerrestrialCoordinates.h.

Geographic coordinates are related to sky-fixed equatorial coordinates by specifying the counterclockwise angle to the prime meridian from the reference meridian $\Upsilon$ of the sky-fixed coordinates, as defined below. This angle is called the Greenwich Mean Sidereal Time (GMST), and is often specified in hours, minutes, and seconds.

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$\parbox{0.41\textwidth}{\caption{Definition of the ecliptic sky-fixed coordinate systems.}}$