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From: Henry C. Halboth (no email)
Date: Thu Apr 07 2005 - 11:44:01 EDT
George,
I am sorry to find that I misspoke regarding Arnold and the subject
matter. He does briefly, c. 1820, speak of determining LAT by
sunset/sunrise, and specifically alludes to use of the sunset/sunrise
tables then in existence for this purpose, including interpolating for
seconds between the whole minutes given therein. He does not speak well
of the method, allowing it to be most inaccurate in Arctic Latitudes but
somewhat less so in mid-latitudes, stating ...
"I high latitudes the time observed by this method will vary from one to
four minutes, depending on the Latitude and the variation of the
refraction." and ...
" Within the Latitude of 35, I have found it , by comparison with
observations had the same afternoon to agree with a good watch,
previously regulated by an altitude of the Sun, within from fifteen to
thirty seconds" ... This would, most roughly, limit accuracy of any
resultant position to about six or seven miles.
There appears little here in the way of the "precision" usually sought by
this group, but in the hands of a navigator desperate for a position (a
situation I well understand) it is another trick in the bag, to be used
only with careful evaluation and consideration to the circumstances of
time and place - as is the case with many of the older methods - after
all, if you could get a position within the visibility distance from the
foretop you were doing pretty good on many an occasion. I would be more
inclined, however, to solve this problem by utilizing a corrected
negative altitude and the Time Sight solution.
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 , 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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