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Andes Ruiz wrote-

A very interesting book is available online:



Background to Discovery: Pacific Exploration from Dampier to Cook.

Berkeley University of California Press,  c1990 1990

Howse, Derek, editor.

http://content.cdlib.org/xtf/view?docId=ft3489n8kn&chunk.id=0&doc.view=print
(print view - 147 pages)

http://ark.cdlib.org/ark:/13030/ft3489n8kn/



The chapter "V. Navigation and Astronomy in the Voyages", (14 pages), speaks
about the use of the Lunar Distances method at the beginning of the XVIII
century

========================

I agree with Andres about that chapter. Each chapter is by a different
expert, and that one (pages 160 to 184) is by Derek Howse, who also edited
the volume, and really knew what he was talking about where navigation was
concerned. Other works by Howse, worth reading, are "Greenwich Time and the
Longitude" (National Maritime Museum, 1997), and his section "The lunar
distance method of measuring longitude", in "The Quest for Longitude", ed.
W. Andrewes, (Harvard 1996).

What is especially interesting, in the chapter which Andres recommended, is
Howse's analysis of the going of the various timepieces taken on Cook's 2nd
and 3rd voyages, which shows us the extent to which they required correction
from astronomical observations. I haven't seen that study published
elsewhere. It is indeed quite eye-opening, and corrects the widely-held view
that possession of a timepiece was the complete answer to Cook's longitude
problems. For a short ocean crossing of a month or two, many early
chronometers did the job well, and they were particular useful in Cook's
local coastal surveying. Used alone, they simply were not good enough for
protracted voyages of several years, such as Cook's voyages would be; but
used in conjunction with occasional lunars, they were just what he needed.

Even if a timepiece had been carefully rated to keep good time before
departure, an error in its rate, characteristally of the order of 10 seconds
per day or more, could and did quickly develop, as Howse shows. If this had
remained unchecked, and uncorrected, then over a three year period the
accumulative error in time would build up to something like three hours, or
45 degrees of resulting error in longitude! It was essential, then,  for
corrections to be made at frequent intervals, and Cook would pay particular
attention to that, particularly when he halted for a spell on-land.

Howse is not infallible about lunars, though. Starting on page 174, he
explains the navigator' procedure when taking a lunar, continuing-
"But it is also necessary to measure the altitudes of the two bodies as
nearly simultaneously as possible in order to correct for parallax and
refraction. The lunar distances predicted in the almanac assume the observer
is at the centre of the earth. In fact the observer is some feet above the
earth's surface, and furthermore, both bodies appear too high because of
atmospheric refraction., the amounts depending on their respective
altitrudes ... Both these effects had to be allowed for; indeed, "clearing
the distance", as it was called, was the most laborious part of the whole
computation".

In that passage, Howse is confusing lunar parallax, which is important, with
dip of the horizon, which isn't (not for a lunar, because the horizon isn't
involved). And true to form, by the way, Sobel copied that same error into
her book "Longitude" (page 98 of my 1996 edition).

Now that "Background to discovery" is so accessible, Navlist members
interested in the history of navigation shouldn't miss chapter 5. I found
other chapters less valuable, though chapter 3 (pages 81 to 127) about
French voyaging and technology, was of interest

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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