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Subject: Re: Lunar altitudes
From: George Huxtable (george@XXX.XXX)
Date: Mon Apr 14 2003 - 13:23:39 EDT
Jan Kalivoda wrote-
>The method of lunar altitudes for finding the GMT seems ideal to me for
>the observation above an artificial horizon (ashore, of course). The
>problems with dip, with the blurred sea horizon, with the short period of
>twighlight would then disappear completely. With an artificial horizon one
>can wait through the whole night, until the Moon with a star (not very
>distant from the Moon, but their azimuths don't matter too much in this
>case) come to the prime vertical. Many occasions can arise through the
>month and one could expect better results than with lunar distances, isn't
>it? . This could be very important in early days, when the knowledge of
>the accurate longitude was an exception outside the Europe and USA (maybe
>Caribbean) and the method of combining local time and longitude to obtain
>the GMT was therefore unavailable in remote countries.
>
>Had you anybody heard anything about such usage? I didn't.
>
>Maybe somebody from the list, who is in possession of a sextant and an
>artificial horizon, can try this method? It cannot be too difficult, I
>guess (from the desktop).
>
>
>Jan Kalivoda.
======================
Response from George-
Until chronometers were available that could be trusted to maintain
accuracy long-term, longitude by lunar-distance was a method used by
surveyors and explorers on land, from the 1760s onward. Observers on a
steady land platform had an alternative: with a powerful telescope, they
could get their timimg from the moons of Jupiter: this would be rather more
precise, but available only occasionally.
Noteworthy land expeditions who observed lunar distances included Lewis and
Clark (though they made rather a botch of the job), and Thompson across
Canada.
Rather later was the unsung Augustus Gregory in the Australian outback in
the 1850s, communicated to me by list member Keiran Kelly, who (I hope)
will publish Gregory's celestial methods. Gregory must have been a superb
observer, with a steady hand and a sharp eye, as shown by the following
example- a series of lunar distances, between the Moon and Mars. Just look
at these numbers-
Chronometer time Lunar distance
22h 19m 00s 29° 54' 20"
22h 23m 00s 29° 52' 40"
22h 27m 00s 29° 51' 10"
22h 31m 00s 29° 49' 30"
22h 35m 00s 29° 47' 30"
22h 39m 00s 29° 43' 50"
22h 43m 00s 29° 44' 00"
22h 47m 00s 29° 42' 10"
Just as a table of numbers, these don't mean a lot, but I urge readers to
plot them out on a bit of graph paper, when they come to life, and you can
see what a real master of the Vernier sextant could achieve, and how
precise his averaged result would be. Can any readers, with their modern
micrometer instruments, do as well?
Altitudes of Moon and Mars, reflected in a liquid, were interleaved between
these lunar distances.
There is no record of Gregory carrying lamps or candles on his long
journey, and it seems likely that the Vernier had to be read by the
flickering light of his camp-fire.
It's a lesson to us all.
These methods are the way the world was mapped.
George Huxtable.
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contact George Huxtable by email at george@XXX.XXX 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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