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    Re: Plumb-line horizon vs. geocentric horizon
    From: Jim Thompson
    Date: 2005 Feb 10, 09:02 -0400

    > -----Original Message-----
    > From: Pierre Brial
    > ...maybe you could evaluate the difference of geoid height above the
    ellipso?d on your place and on the horizon, and add this difference to the
    eye height for your computation...
    Interesting thought.  Nice to have your geodesy expertise on the list,
    Pierre.  I am very new to all this.
    The geoid varies from the ellipse around the world by up to + 73 m in New
    Guinea, and - 105 m off the coast of southern India. Those would be
    significant heights of eye.  But is it necessary to consider this in
    splitting-hair celestial navigation (putting aside the obvious point that
    the differences are not practical for most CN)?  Are the Nautical Alamanc's
    data referenced to the earth's ellipse, or to a model based on the geoid?
    Canada's geoid is only +/- 5 cm off the center of the earth (Natural
    Resources Canada website today).
    Here is my current understanding (my own words, not an authority):
    "Astronomical latitude is the  angular distance between the plumb line and
    and the celestial equator. Since the celestial equator is coincident with
    the geoid's equator, then astronomical latitudes are the same as terrestrial
    latitudes. Geodetic latitudes plotted on an accurate model of the geoid will
    very closely match astronomical latitudes. This is the critical connection
    between a sextant observation, the astronomical data in the Almanac, and
    your chart: If your chart is based on a sound model of the geoid in the
    region you are sailing, then a latitude determined from a sextant sight will
    be nearly the same as the latitude on your chart.".
    But I am still not clear on the coordinate reference model on which the
    Nautical Almanac is based.

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