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    Re: Visibility at rising and setting
    From: Philip Ouvry
    Date: 2000 Apr 25, 11:00 AM

    This sort of theoretical aspect of astro-navigation comes up from time to time
    but is really completely impractible.   It depends on how well you can see the
    horizon in dark conditions.   It is a similar problem to taking star sights
    using the light of the moon to illuminate the horizon.   False horizons are a
    strong possibility.
    It is rather similar to the problem of navigation whislt adrift in a liferaft
    with nothing but a watch and a simple compass.   By observing when a planet or
    major star such as Sirius rises and sets each day it is possible to forecast the
    exact moment and position on the horizon of the rising and setting.   By
    measuring the amount by which the rising or setting is early or late one can
    work out the distance run over 24 hours or so and hence ones approximate
    longitude.   Latitude is a bit more difficult without some way of measuring
    Philip Ouvry

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