The astronomical latitude (Φ) is defined as the angle between the equatorial plane and the true vertical at a point on the surface: the true vertical, the direction of a plumb line, is the direction of the gravity field at that point. (The gravity field is the resultant of the gravitational acceleration and the centrifugal acceleration at that point. See Torge.) The astronomic latitude is readily accessible by measuring the angle between the zenith and the celestial pole or alternatively the angle between the zenith and the sun when the elevation of the latter is obtained from the almanac.
In general the direction of the true vertical at a point on the surface does not coincide with the either the normal to the reference ellipsoid or the normal to the geoid. The deflections between the astronomic and geodetic normals are only of the order of a few seconds of arc but they are nevertheless important in high precision geodesy.
Astronomical latitude is not to be confused with declination, the coordinate astronomers used in a simlilar way to describe the locations of stars north/south of the celestial equator (see equatorial coordinates), nor with ecliptic latitude, the coordinate that astronomers use to describe the locations of stars north/south of the ecliptic (see ecliptic coordinates).
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