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George,

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
explanation.


  Henry

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@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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