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George,

I have really very little to go on, but would personally consider the
final disappearance of the Sun's upper limb below the visible horizon as
the visible moment of apparent sunset, thus neglecting the effects of
refraction, height of eye, etc., and to which one would, of course, apply
the usual corrections to obtain a true altitude for any subsequent
calculations.

While writing this, I broke out HO 220 and can quote therefrom to the
effect that: "Sunset = the crossing of the visible horizon by the upper
limb of the descending Sun." The same definition applies to Sunrise - it
is the appearance/disappearance of the upper limb that counts.

I have nearer come across any table to facilitate the calculation of
Longitude specifically at sunset/sunrise, but there have been and are
short tabular methods to solve the Time Sight, which supposedly could be
extended to include a negative altitude - Martelli's, for example, are
calculated to an altitude of 0-degrees and could easily be extended, but
these are also c. 1900 and do not provide an answer to your question. My
friend Arnold does not deal with this problem and I otherwise think that
you or Fred would have access to any other tables that I might have..

By the way, Martelli provides an interesting solution in which I think
you would be interested, if not already familiar therewith. It seems
readily calculator friendly, although does not provide azimuth which can
be easily added.


       Henry

On Thu, 7 Apr 2005 00:45:14 +0100 George Huxtable
 writes:
> Henry Halboth wrote-
>
> >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.
>
> ===================
>
> Thanks, Henry.
>
> Yes, I agree, there's no problem in calculating that hour-angle from
> spherical trig. I can do it, and so, no doubt, could Scoresby, if he
> had
> to.
>
> It's just that he says specifically that he got it from a table that
> makes
> me wonder what that table was.
>
> I agree, too, that it's perfectly possible to use upper or lower Sun
> limb,
> against the horizon, for a time-sight, or perhaps both alternately.
> I ask
> Henry which of these a professional mariner would refer to as "the
> moment
> of sunset".
>
> 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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