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George H, you've referred to Halley's observations as "lunars". I don't think
that's necessarily a good idea. They're conjunction and occultation
observations. These were widely used by astronomers starting around Halley's time, but
I bet he was one of the very few to observe them at sea. The Nautical Almanacs
in the late 18th century included a table of "Other Phenomena" for every
month that listed conjunctions of stars with the Moon. Many of these stars were
quite faint. For example, the almanac for August, 1805 lists a conjunction with
theta Ophiuchi on the 6th at 01:59 Greenwich time. Later in the month, on the
17th at 15:13 there's a conjunction with the star they called eta Pleiadum
(eta Tauri today).
The process of reducing observations of occultations and conjunctions was
considered much more complicated than lunars proper, and so it was never
considered a real "navigational" method of finding longitude but definitely useful in
the hands of a skilled astronomer. Also, the horns of the Moon can be
decidedly variable since the mountains and craters at the poles can hide or expose
large areas. It would be an interesting modern experiment to see how accurately
one can deduce Greenwich time by watching faint stars line up with the horns of
the Moon through a low-power telescope (such as might be held stable on a
vessel at sea). Personally, I doubt that this could be done better than about
five minutes of time. With proper astronomical instruments and detailed
calculations, of course, it can be very accurate.
Speaking of those craters and mountains and their variable orientation, they
make the limb of the Moon "bumpy". It seems to me that the relative
unpredictability of the mountains on the Moon's limb would place the ultimate limit on
lunars. From photos I've taken, I would estimate that the mountains and valleys
along the limb make a zone about 0.05 minutes of arc thick. So the sextant's
ability to measure angles down to 0.1 minutes is really very close to the
ultimate limit of lunar distances you could ever measure. Can anyone put better
numbers on this issue?
Frank R
[X] Mystic, Connecticut
[ ] Chicago, Illinois
