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    Cannot dispense with the assumed position at sea
    From: Jim Thompson
    Date: 2004 Feb 18, 13:19 -0400

    I did a hundred Neptunic hail marys for failing to realize that an AP is
    essential at sea.  Now how does this look? :
    There are 5 essential pieces of data for reducing a celestial sight to
    plottable information:
    1. Observed altitude of the Body above the celestial horizon. Measure it
    with a sextant (hs), and then apply relevant corrections to get Ho.
    2. Latitude and longitude of a nearby assumed position (AP).
    3. Precise time of the sextant altitude measurement, in order to find Hc and
    Zn for the nearby assumed position (AP).
    4. Computed altitude of the Body (Hc) as if observed from the AP at the time
    of the sextant sight. Requires Almanac ephemerides.
    5. Bearing of the Body (azimuth). One can only determine the azimuth for the
    AP, not for the vessel's real position; so the navigator needs to be precise
    about the time of the sextant altitude, and reasonably confidents in the AP.
    Requires Almanac ephemerides.
    1 comes from the sextant sighting and Almanac data.
    2 comes from the DR plot.
    3 comes from a timepiece simultaneous to #1.
    4 and 5 come from calculations to solve the navigational triangle with
    corners GP, AP and nearest Pole, using #2, #3 and Almanac data.
    Without those five pieces of information, nothing useful can be plotted.
    Can we plot a celestial sight if we do not have a nearby "assumed position"?
    Not at sea. A timed sextant sight by itself, without a reliable AP, can only
    tell you how far away you are from the GP of the Body on a circular line of
    position often many thousands of miles in circumference. You need a bearing
    to the GP in order to find out where you are on that LOP (COP), but such a
    bearing cannot be measured accurately at sea, and cannot be obtained
    magically from Almanacs or computers without more information about your
    position. There are ways to measure azimuth on land, with the right tools.
    So at sea you have to use a nearby assumed position.
    A celestial EP will only be as good as the AP. The more one has to guess at
    the AP, then the less confident one can be about estimating where the vessel
    is on the celestial LOP.
    Jim Thompson
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