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Frank mentioned, in a passage about another topic (Subject: [NavList 2918]
Re: Self Sufficiency) -

"I've recently been re-reading Maskelyne's "British Mariner's Guide to the
Discovery of the Longitude at Sea and Land" (1763) "

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That's a remarkable work, containing all sorts of interesting stuff. Very
advanced for its early date of 1763.

Maskelyne impresses me by the clarity and precision of his careful
explanations about a complex matter.

The only error that I've caught is on page 38, where he says-

"... from the observed distance of the sun and moon's nearest limbs ...
subtract the sum of the semidiameters of the sun and moon; this gives the
apparent distance of the centres of the sun and moon." For "subtract", one
should read "add", of course.

The tables at the back contain, with much other stuff, data for predicting
the position of Sun and Moon, in condensed form, from 1760 to 1780. But the
procedures for making those calculations, given in chap IV, (particularly in
part 2 for the Moon) are mind-bogglingly complex. They were based on Mayer's
observations and analysis. It would be an exceptional navigator who could
meet the challenge of making those calculations at sea, though some managed
to do so. Notably John Harrison (not the John Harrison of the clock), purser
on Dolphin during her circumnavigation, who deduced an accurate longitute
for Tahiti in 1766, this leading the way for Cook three years later.
Presumably, Harrison was working from the British Mariner's Guide.

I have worked my own way through an example, but not without some
false-paths and errors on the way; knowing what the right answer had to be,
made all the difference. So I can appreciate Harrison's achievement, in
getting the right answer for Tahiti, with no way of cross-checking it. Frank
may find it of some interest to try such a prediction for himself.

Indeed, an interesting exercise, for any skilled computer whiz with time on
his hands, might be to download those tables into a spreadsheet, and then
automate that prediction procedure, which involves lots of lookups and
interpolations. The results, for various dates between 1760 and 1780, could
be compared with modern back-predictions for those dates.

That task was done in the later Nautical Almanac, starting in 1767, in which
two (human) computers, unaware of each other's results, were given the job
of calculating Moon positions at 12-hour intervals throughout the year,
which were then interpolated to three-hour intervals. Lunar distances were
then pre-computed from those results.

There's some unexpected stuff in the Mariner's Guide, including advice to
mariners about the correct spacing of the knots in a log-line, and the
timing of a sand-glass to go with it; a matter about which much confusion
existed in 1763.

George.

contact George Huxtable at george@huxtable.u-net.com
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.



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