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On 3 Apr 2005 at 18:26, George Huxtable wrote:

> 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.

David Thompson used the locations that he calculated in the field for his
map of Western Canada and from re-working some of his calculations I can
attest that he took the values published in the Nautical Almanac (many of
which were in error by up to 30" for lunar distances). If Greenwich had
published corrections then Thompson would have been ideally placed to know
about them and use them. His teacher, Philip Turnor, became one of
Maskelyne's computers when he returned to England. Since Turnor would have
been aware of the innacuracies in the almanac, and the impact on
Thompson's mapping, it seems likely he would have informed Thompson of any
sort of erratum that might have been used retrospectively to correct his
longitudes.

If anyone knows of an example of an explorer retrospectively correcting
their longitudes, I too would like to hear about it.

Ken Muldrew.

   

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