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From: George Huxtable (no email)
Date: Fri Dec 09 2005 - 12:47:07 EST
I had written-
> "What Baffin needed to know was the hour, in Apparent Time at London, at
> which the Moon was predicted to cross the Meridian of London, on the same
> day that he measured the corresponding Local Apparent Time in Greenland
> of
> the Moon crossing the meridian there."
And Frank replied-
> What's interesting, too, is that Baffin didn't need to know this
> information
> on the spot. Rather than using a pre-calculated ephemeris, his
> observations
> could have been rendered much more useful if he had had an accomplice (or
> several actually) back in England carefully observing the Moon's position
> as
> often as possible --a makeshift Royal Observatory. Then, even if the
> ephemerides
> of the era were poor, the longitude based on his observations would have
> been as accurate as the best observations of the era. Who knows... maybe
> someone
> even thought of that after Baffin returned home.
>
> An accurate lunar ephemeris, calculated in advance, is a necessity for
> "live" navigation by lunar position/lunar distances to a known
> destination, but
> it's merely a convenient luxury for surveying and exploration. For
> mapping the
> world, what's required is nearly simultaneous observations from a known
> longitude. The Moon can do what it will, and we don't need to have the
> science to
> predict it if our purpose is only mapping.
===================
Response from George
It's all very well for Frank to suggest such a procedure, but we need to be
aware of the early dates of these Baffin voyages, 1612 and 1615.
It would be necessary, as Frank suggests, to have a working observatory,
back at "home base", to systematically measure Moon positions, night after
night, over a long period of many months (perhaps years), just in case
Baffin happened to be measuring the Moon on that same night, on the other
side of the Atlantic..
There are several problems about that. One is the climate of London. On
about two nights in three, cloud cover spoils an observation of Moon's
transit in London, and I doubt it was much better in the 1600s, so
interpolation, between observations on "good" nights, would be needed.
Greenwich Observatory , which was built on a hill outside London to get
clearer air, was still 60 years away. There was in Baffin's time nobody
making really systematic sky-observations in London, that I know of.
In Paris, 60 years after Baffin, there would be fruitful collaboration after
the event, along similar lines to Frank's suggesttion, between surveyors and
astronomers. But that relied on a permanent observing team at Paris
Observatory, mainly using Jupiter satellites.
The ephemeris used by Baffin, produced by Searle, seems to have been made
for astrological purposes (just celestial longitudes, no celestial latitudes
or declinations) and acknowledges Regiomontanus and also the
recently-deceased Tycho. Tycho had been making by far the best systematic
observations of that era. I can see no evidence that Searle had made
observations of his own, and presume that the numbers had all come from the
Tycho - Kepler collaboration, simply adapted by Searle for the longitude of
London.
Yes, a "makeshift Royal Observatory", in Frank's words, could have been
useful for Baffin's mapping, but the time wasn't ripe for it then.
An explorer, discovering new lands, didn't need to know, for navigation
purposes, where he was, in terms of lat. and long. He had no chart to
follow; any chart would be made by him, so filling in lat and long was a job
that could be done later, using hindsight if necessary. Baffin, in Hudson
Bay, wasn't first on the scene. Hudson had been there before him, but had
left such limited information that Baffin was pretty well starting afresh.
George.
contact George Huxtable at
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
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