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    Re: Lunars for dummies like me
    From: George Huxtable
    Date: 2004 Sep 25, 00:15 +0100

    To the question-
    >>But how did they convert the difference between LAN and LAT to a time for
    >>their ship's clock?
    Zorbec Legras responded-
    >In the French Navy they did it this way:
    >1? Take the local time at apparant sunrise from the ship's clock (Tvge)
    >2? Compute P for altitude = 0, with cos P = tan lat tan dec.
    >(table 24 Friocourt)
    >3? 12 - p = local time Tvg
    >4? Tvg - Tvge = correction to apply to the ship's clock.
    >The ship's clock was not used for nautical computations
    George's comment-
    Well, as long as the ship's clock was NOT used for nautical computations,
    that might be adequate enough timing for ringing the ship's bell. But it
    was a crude and inaccurate procedure.
    The moment of sunrise is very dependent on refraction in the lower layers
    of the Earth's atmosphere, within a few metres of sea level, which varies
    significantly depending on local weather. For that reason navigators are
    advised to confine their observations to bodies of more than 10 degrees in
    altitude, in which case the refraction is much less and much less variable.
    In this context, I recommend a recent thorough and authoritative paper by
    Andrew T Young, "Sunset Science IV. Low altitude refraction." in The
    Astronomical Journal, vol 127, pages 3622 - 3637, 2004 June. Meeus'
    "Astronomical Algorithms" refers to a paper by B E Scheafer and W Liller,
    "Refraction near the Horizon" (Publ. Astron. Society of the Pacific, vol
    102, pages 796-805, July 1990), which I haven't read. Meeus quotes them as
    saying that refraction at the horizon fluctuates by 0.3 deg around a mean
    value normally, and in some cases apparently much more.
    But setting aside the FLUCTUATIONS in refraction, how do the French define
    the moment of sunrise for this purpose of setting the clock, in view of the
    big effects of MEAN refraction?
    From Zorbec's account, it seems that the tables he mentions refer to the
    moment when the true altitude of the Sun is zero. In that case, the centre
    of the Sun would, because of refraction, appear to be elevated by nearly a
    Sun-diameter. Hardly the moment of sunrise as most of us would define it.
    Most of us would define sunrise as the moment when the Sun's upper limb
    peeps over the horizon. At that moment the true direction of the Sun's
    centre is still about 50 arc-minutes below the horizon.
    So I ask Zorbec, what moment, around sunrise, do French navigators choose
    to time when they adopt this procedure? Do they make any adjustment to
    allow for refraction; or is any such adjustment included in the tables?
    Note that we haven't even considered the effect of dip and the observer's
    height of eye.
    contact George Huxtable by email at george@huxtable.u-net.com, by phone at
    01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
    Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

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