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    Re: Request for help re sunset predictions.
    From: Henry Halboth
    Date: 2005 Apr 6, 16:18 -0400

    I have rather extensive examples of Longitude determination by sunset +
    sunrise which alternately use both upper and lower limbs at the time of
    occurrence. In any example that I have access to the resultant negative
    altitude is used in a conventional Time Sight solution. These examples
    are c. 1900 and make no reference to special tables and, to me at least,
    there seems to be little need for any such as the solution is relatively
    simple.
    
    On Wed, 6 Apr 2005 20:05:23 +0100 George Huxtable
     writes:
    > 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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