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George H wrote:
"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."
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.
-FER
42.0N 87.7W, or 41.4N 72.1W.
www.HistoricalAtlas.com/lunars
