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    Re: What is a degree of latitude?
    From: Paul Hirose
    Date: 2008 Mar 24, 11:41 -0700

    Geodetic latitude is used in mapping and navigation. It's the angle
    between the equatorial plane and a line that passes through the point of
    interest and is normal (perpendicular) to the ellipsoid. It's also the
    declination at which the normal intersects the celestial sphere. E.g.,
    at 40 degrees north latitude, +40 degrees declination is at your zenith.
    This is why celestial sight reduction can use spherical trig and ignore
    the ellipsoidal shape of Earth. In principle, the computation is done on
    the celestial sphere.
    Strictly speaking, there something else to consider: the deflection of
    the vertical. Since Earth is not homogenous, its gravity field is
    slightly irregular. A vertical referenced to gravity will therefore not
    exactly coincide with the ellipsoidal normal through the same point.
    Observations based on a gravity vertical give astronomic (not geodetic)
    latitude and longitude. The discrepancy is normally just a few seconds
    and is insignificant compared to other errors in a sextant observation.
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