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    Re: Request for help re sunset predictions.
    From: Henry Halboth
    Date: 2005 Apr 6, 23:02 -0400

    This is a long shot - but do you suppose he could have been making some
    use of Norie's Table XLV - "For finding the time most advantageous for
    observing the altitude of a celestial object, in order to obtain the
    apparent time" - by considering sunset as the Sun's closest approach to
    the Prime Vertical and working backwards by interpolation to arrive at a
    time. This table, entered with Latitude + Declination,  gives time in
    hours + minutes before or after apparent noon, but could be interpolated
    to seconds. I an not advocating this methodology, but it might lead to an
    On Wed, 6 Apr 2005 20:05:23 +0100 George Huxtable
    > I'm following some whaling journals of William Scoresby the younger,
    > who
    > visited the Greenland Sea (West of Spitzbergen) each year from 1811.
    > Some of his time-sights, to determine LAT, were taken by observing
    > the
    > moment of sunset. I take that to be defined by the last glimpse of
    > the
    > Sun's upper limb above the horizon. Does anyone think differently?
    > To my mind, it's a poor choice of moment to determine time, when the
    > Sun's
    > centre appears to be actually below the horizon, and refraction
    > corrections
    > are large, and rather variable. However, that was what he did, on
    > occasion.
    > It saved the trouble of getting his sextant out, no doubt.
    > He appears to have obtained his local time, at the moment of sunset,
    > from
    > tables into which he entered lat and dec, quoting a resulting time
    > of
    > sunset to the second, e.g. "6h 13m 28s pm".
    > Does anyone know where such tables were to be found, by a navigator
    > in 1811?
    > My earliest such compendium is Raper's "Practice of Navigation",
    > 1864, in
    > which table 26 is "apparent time of the Sun's rising and setting",
    > tabulating lat at intervals of 1deg, but dec at intervals of 2deg,
    > and
    > giving a time to the nearest minute. Not nearly good enough for
    > interpolating a result to the nearest second. Not only that, the
    > time of
    > sunset, for all lats, when the dec is exactly zero, is given as
    > exactly
    > 6pm. That would only be true for a star (with no semidiameter) and
    > if the
    > refraction and dip were exactly zero: or if all three quantities
    > cancelled
    > out to zero. It seems that Raper's table 26 is intended to give no
    > more
    > than a rough notion of time of sunset, good enough for many
    > purposes, but
    > not for a time-sight.
    > I also have an edition of Norie's, tables dating from 1914, which
    > gives
    > table XLIII (43), "semidiurnal and seminocturnal arcs" , giving
    > times from
    > noon to sunset to the nearest minute, and in this case the decs are
    > tabulated in intervals of 1 degree. But this is claimed to handle
    > "any
    > celestial object", and there's no provision to insert a Sun
    > semidiameter,
    > so presumably this table also isn't intended to give any precise
    > timing for
    > the moment of sunset.
    > So I ask any Nav-L members, who own or have access to navigation
    > tables for
    > the early 19th century, whether they can identify any table,
    > anywhere, of
    > sunrises/sunsets, that Scoresby might have used to get his LHA, in
    > 1811 and
    > following years.
    > George.
    > ================================================================
    > contact George Huxtable by email at george---.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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