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From: George Huxtable (no email)
Date: Wed Apr 06 2005 - 15:05:23 EDT
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.
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contact George Huxtable by email at , 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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