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    Visibility at rising and setting
    From: George Huxtable
    Date: 2000 Apr 25, 9:24 AM

    The May issue of the UK magazine Practical Boat Owner carries a short
    article entitled "sights without a sextant" by Alastair Buchan. This
    suggests that you can make worthwhile astro observations of Sun or Moon,
    perhaps even stars and planets, to obtain a position line without a
    sextant, by timing the moment at which they are tangent to the horizon at
    rising or setting.
    There are well-known weaknesses in this approach, which I propose to write
    in to point out.
    However there is one claim made that goes way outside my own experience,
    and perhaps I can call on the collective wisdom of this mailing list for
    help. The author states "I've never tried it myself, but given a dark sky
    and a clear hard horizon, it should be possible to take horizon sights of
    stars and planets - although deciding when they're exactly at tangency may
    involve more of a guess than observation."
    Clearly, there's a special problem at rising, in that the star or planet is
    invisible before rising, and you have little idea beforehand just when and
    whre it's going to happen, so let's confine ourselves to settings.
    What I'm asking is whether any of you enjoys clear enough skies that they
    can observe stars or planets, even Venus at its brightest, right down to
    being able to time a sudden moment of extinction as it sets below the
    horizon. As a navigator in the waters of NW Europe, with its notably hazy
    horizons, I am rather at a disadvantage in this respect. I am somewhat
    sceptical that it is ever feasible elsewhere, but I look forward to
    receiving the comments of others. Presumably, one could be certain that the
    light from star or planet really was being intercepted by the now-dark
    horizon itself, if it were being occulted on and off a few times, by
    distant waves or slight swell, as it set, similar to observing  a distant
     I am setting aside, for the moment, the unpredictability of the refraction
    correction which bedevils all low-altitude observations.
    Yours, George Huxtable
    George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    Tel, or fax, to 01865 820222 or (int.) +44 1865 820222.

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