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George,

Quoting from Norie, 1839 edition, regarding amplitudes ... "The
observation should be made when the Sun's lower limb appears somewhat
more than his semidiameter above the horizon ..." etc.
If I search long enough, there probably is a reference to a full diameter
somewhere, always bearing in mind the old saying "different ships
different long splices", as well as the simple fact that you don't learn
practical navigation from a book - only the bare technical essentials.

On Mon, 2 May 2005 10:30:09 +0100 George Huxtable
 writes:
> Henry Halboth responded to Kieran Kelly's request included this
> comment-
>
> If observed with Sun's center on the
> >visible horizon as at sea, a correction for refraction + parallax +
> dip
> >(if applicable) would technically be necessary; it was, however,
> >customary for seagoing navigators to observe the Sun when a
> diameter
> >above the horizon and to ignore this correction - certainly not
> "precise"
> >but good enough in ascertaining compass error at sea. One may also
> refer
> >to Tables 27 + 28 in Bowditch and achieve the same result by
> >interpolation.
>
> That's interesting. When Henry says that navigators "observe the Sun
> when a
> diameter above the horizon", what part of the Sun disc does that
> refer to?
> If it's the Sun's centre, that would correspond reasonably closely
> with the
> statement in the Admiralty Navigation Manual, vol II, that at the
> moment of
> Theoretical Sunset, when the true altitude of the Sun's centre was
> at 0deg
> 0', the altitude of the Sun's lower limb would be 16 arc-minutes.
>
> Presumably, if a closer approximation to theoretical sunset was
> wanted, an
> observer could also allow for his height of eye, adding say 4
> minutes to
> the altitude of the observed Sun (perhaps just by estimation) if his
> height
> above sea level was 16 feet.
>
> However, there's never going to be any great precision in estimating
> the
> moment of theoretical sunset that way, because the refraction near
> the
> horizon is so great, and so variable.
>
> It seems, then, that we have two very different "moments of sunset".
> There's the dictionary definition, the instant when the last rays
> twinkle
> at the horizon, which is what the sunset predictions at 3-day
> intervals in
> the Nautical Almanac refer to (though perforce they omit any
> correction for
> height of eye, not knowing what that will be). And there's
> Theoretical
> Sunset, when the true altitude of the Sun's centre is zero, and the
> Sun's
> centre actually appears to be a diameter (or more, depending on dip)
> above
> the horizon. It's this latter moment, of theoretical sunset, that's
> been
> used for tables of sunset times and amplitudes in several epitomes,
> such as
> Raper and Norie, without stating that important distinction clearly,
> or at
> all.
>
> Henry says that the method of estimation he describes was customary
> for
> navigators, and I believe him. But I haven't seen it referred to in
> textbooks, and I wonder whether it was ever advocated in formal
> advice to
> mariners.
>
> =========================================
> =========================================
>
> This may be relevant to my earlier question of April 6th, "Request
> for help
> re sunset predictions." to which Henry, and Dan Allen, have
> responded.
>
> Here it is, again.
>
> ===============
> "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."
> =================
>
> If Scoresby chose to time for sunset, not the moment of the last
> twinkle,
> but the moment at which the Sun's centre was, say, a diameter above
> the
> horizon (perhaps estimated by eye, and perhaps including an extra
> allowance
> for dip depending on his height of eye), then the Sunset tables in
> Raper
> and Norie would give him the right answer. Or would, if refraction
> at the
> horizon was actually at its predicted "mean value".
>
> So, I ask, would a navigater of the day (1811) be familiar with the
> "dodge"
> that Henry describes above, and accustomed to estimating, by eye, at
> just
> what moment the Sun passes through that particular altitude?
>
> As I noted earlier, it's not, in any way, the ideal moment to take a
> time-sight.
>
> 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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