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    Re: Please tell me if I have this right
    From: George Huxtable
    Date: 2006 Apr 19, 22:18 +0100

    Greg asked-
    | To the List,
    |   All - when doing a time sight at sea in period say circa 1780,
    | I seem to have gleaned "the latitude used was the estimated latitude"?
    | I assume this means latitude from a noon sight was applied (with DR
    | corrections)to the time sight that was done 2-3 hours before or after
    | the noon sight? while they certainly would not have traveled far at
    | say 6 to 12 knots some error would have undoubtably crept in due to
    | unknown currents etc. and it would not be impossible for the actual
    | lattitude to be off by 2-4 miles. How did they compensate for this
    | any ideas?
    You're right, as is Robert Gainer when he commented-
    "I would think that if you don't know the amount of error (drift from unknown 
    current) you couldn't build in a correction.  The
    unknown is just that, unknown."
    It was a limiting factor. But not much of a limiting factor, in practice. 
    Remember, in 1780 there were no more than a handful of
    experimental (and temperamental) chronometers at sea. Longitude navigation, if 
    practiced at all, was done by lunars. Because of the
    slow motion of the Moon across the sky, even if a lunar distance could be 
    established to 1 arc-minute, and even if the Lunar tables
    were accurate (which they weren't), the resulting error in Greenwich time 
    would give rise to a longitude error of about 30
    arc-minutes. Compared with that, the error in estimating change in latitude 
    since noon, which was estimated to be 2-4 miles, was
    rather insignificant. If a time-sight could be taken when the Sun was near to 
    due East or West (possible in Summer, not Winter) then
    errors in estimated latitude had little or no effect.
    Peter Fogg commented-
    "The major ocean currents are well known, although of varying speed. Tidal
    movements around coasts and counter currents (running in the opposite
    direction to the main one close to the coast) can be bafflingly
    I think that was somewhat off the mark. The question was about circa 1780, 
    when ocean currents were only very imperfectly
    understood.. It was mostly by the work of Maury, US navy, in the mid 1800s, 
    that navigators became more enlightened about ocean
    currents. Peter is right about the tides.

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