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Frank Reed wrote-

|
| Regarding Halley's calculational method for determining the Moon's true
| position... In fact, he didn't need one as long as he had a diligent
| astronomer back home willing to measure the Moon's position as frequently
as
| possible. For mapping and surveying, where the position determined by
| observations can wait to be calculated when we get home (as opposed to
live
| navigation where we want the position on the spot), we don't need any
lunar
| theory at all.

That is indeed perfectly true (though not for navigation, as Frank
recognises). Than was the basis on which Cassini and the French surveyors
collaborated to remap the Kingdom of France, using mostly Jupiter satellite
events. It was how James obtained a good longitude for James Bay (off Hudson
Bay) as early as 1631, by timing a Moon eclipse. After his return, he could
compare it with the same event seen in England, which he had arranged for
his friend, the astronomer Gellibrand, to observe if the London sky was
clear (it was). It called for a cooperative astronomer back home.
Unfortunately, Flamsteed, the Astronomer Royal at the time of Halley's
voyages, was not made in that mould. His Moon observations at Greenwich were
only sporadic, rather that systematic, and he kept them close to his chest
for as long as he could possibly could; to the despair of Newton, who
desparately needed them to inform his calculations.

However, Halley was not working his longitudes retrospectively, though
that's all he would need to make his chart of variation when back home. He
was actually using those longitudes, on the spot, in real-time, to direct
his navigation, so Frank's suggestion did not apply. The very first lunar
appulse that he records, for 15 Feb (Julian) 1699 appears in his journal as
follows (from ed. Thrower, "The three voyages of Edmond Halley ...", Hakluyt
Society, 1981, page 98 & footnote 5)

"This morning I observed the Moon apply to a starr in fascia sagittari ii
boreali and conclude myself 160 leagues more westerly than our account [i.e.
his DR position], and but 50 leagues to the East of Fernando Loronha". And
indeed, he arrived in Fernando Noronha on the 17th, so Halley's longitude
wasn't far out. And remember, it wasn't done by measuring lunar distance,
but by simply looking with a telescope, five or six feet long, at the Moon's
precise position with respect to that star, as they closely passed. And this
was 68 years before the first Nautical Almanac appeared, with its lunar
distances.

If course, there was much guesswork in such navigation, as charted coasts
and islands had great errors in their mapping, and Halley was able to report
to the Royal Society on his return that Brazil had been charted a degree out
in its longitude.

In my view, Halley has never been given due credit, as the first mariner to
measure longitudes at sea, on a proper scientific basis.

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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