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    Re: Thomas Jefferson and Lunar Obs.
    From: George Huxtable
    Date: 2005 Apr 3, 18:26 +0100

    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
    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.
    contact George Huxtable by email at george---.u-net.com, 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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