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Frank Reed wrote, about observations for longitude made by land travellers-

> ...The almanac is not required for clearing any lunar
>observations for longitude UNLESS you need the longitudes en route (as aboard a
>ship at sea heading for a destination with a known latitude and longitude).
>We've discussed this recently. For exploration and mapping where the
>longitudes  en route are not critically necessary, what you need most of
>all are good
>observatory observations (e.g. at Greenwich) covering the period of the
>observations made in the field. This was the fundamental issue that seems
>to  have
>escaped those early American seekers of longitude.

==================

That's perfectly true. Indeed, Cassini, at the Paris observatory, had a
valuable collaboration with French surveyors; particularly with regard to
the timing, by observation rather than prediction, of disappearences of the
inner satellites of Jupiter.

Did Greenwich provide that sort of service, I wonder, for their observed
lunar transits, in the period we were discussing (around the time of the
Lewis and Clark return in 1806) ?

I don't recall seeing any accounts of travellers being assisted by the
observatory after their return, to correct lunar-based or Jupiter-based
longitudes for errors in prediction. That's not to say it didn't happen,
and I would welcome any references to it, if it did.

There are some difficulties in travellers relying on after-the event
correction of times of lunar transit, from observations of when that same
transit occurred at Greenwich.

One difficulty arises from the English weather. Not only did the sky have
to provide a clear view of the Moon, at the moment of local transit for the
traveller, but to obtain a time correction from Greenwich, the sky would
also have to be clear, at the moment of that transit, at the observatory.
Somewhat less than half such events could be observed at Greenwich: the
rest were obscured by cloud, which can often obscure British skies for a
week or more at a time. So the astute traveller, if relying on such
observatory confirmation, would need to stay at the same spot for many
nights remeasuring his lunar transits, to reduce the risk of the same
transit being missed at Greenwich.

Some assistance may be got from the timing of a transit from another day,
closely adjacent to that being observed, but the longer that time-interval,
the more the result will depend on lunar theory rather than simple lunar
observation.

Another difficulty arises from international politics of the time. Would
American travellers, departing in 1803, be able to rely with confidence on
future cooperation from the British Commissioners of Longitude? True, just
at that time there was a fragile peace in Europe, signed at the Treaty of
Amiens. But what was going to be the British Admiralty attitude to this
upstart nation of ex-colonials, recently in alliance with the great enemy,
Napoleon, and who had recently concluded a major deal with him (the
Louisiana purchase)?

I understand that available in America at that time was a local edition of
the Nautical Almanac, and I would be interested to learn whether this was
produced by agreement with the Board of Longitude, or whether it was a
pirated copy. Such matters may illustrate the state of affairs between our
two nations at that time. Indeed, Paris observatory, rather than Greenwich,
may have provided a more cooperative source for such lunar information, and
the longitude difference between the two observatories was, by then, very
precisely known.

Much later, the Greenwich astronomers assembled a back-catalogue of actual
past lunar observations. There are some interesting references in Eric
Forbes' booklet, "The birth of Navigational Science", which is No. 10
(1974) in a series "Maritime Monographs and Reports" by the National
Maritime Museum, Greenwich: however, I have not (yet) got round to
following them up. I understand that Forbes has provided further details in
a paper "The foundation and early development of the Nautical Almanac", in
Journal for the Institute of Navigation (now "The Journal of Navigation"),
xviii, (1965), 391 -401.

It appears that Thomas Young (the wave-theory of light man) produced a
"Report on the Progressive Improvements of the Lunar Tables", which is
preserved among the Board of Longitude's Confirmed Minutes for 2 November
1820, and contains a synopsis of more than 4000 observations of the Moon's
celestial position over a period of 36 years. The reference given is - RGO
MSS PRO ref 535, p. 317. The RGO (Royal Greenwich Observatory) archives are
now held within the Cambridge University Library, and their custodian is
the helpful Adam Perkins .

Later, there was published by the Lord Commissioners of the Admiralty,
(1848) "Reduction of Observations of the Moon made at Greenwich from 1750
to 1830.", which sounds rather promising.

These were presumably studies, made long after the event, with a view to
diagnosing and correcting weaknesses in the lunar predictions, and far too
late to be useful to any geographer of Jefferson's time. Indeed, it may
well have been in the observatory's interest to hide, rather than
publicise, discrepancies between their lunar predictions and actual
observations, to make their almanac appear to be more precise than it
really was.

George.

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