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    Re: Wright's 1599 Chart showing Scillies
    From: George Huxtable
    Date: 2007 Nov 30, 21:11 -0000

    I had written, about Wright's longitudes for his zero meridian, Cape Verde
    or the Cape Verde islands
    "Here, I am only guessing. All I have about Wright is what David Waters has
    to say, in "The art of navigation in England in Elizabethan and early
    Stuart.times" (1958). By the way, in plate 58, he shows the same chart of
    the NE Atlantic as Gary has done.
    Waters doesn't seem to say anything about his longitudes. But as I see it,
    the best way of getting longitude, at Wright's date of 1599, would be to
    find the local time of an eclipse of the Moon, a method which went right
    back to Ptolemy. At night, my guess is that the local time could be
    estimated using a nocturnal, based on the angle that the two guide-stars,
    "The Guards", make with the horizon. It would be best if the same eclipse
    had been observed back home, but otherwise, it would have to rely on
    predictions. I doubt if any longitude could be determined to better than a
    degree or two that way, but even so, that would be better than dead
    and Peter Fogg has commented-
    | Given that "a degree or two" amounts to 60 to 120 nautical miles, surely
    | this is a bit of a sweeping generalisation?
    Indeed it it. And it was meant to be. That's why it was larded with such
    words as "only guessing", "my guess is ..."; "a degree or two", in assessing
    the likely error in longitude. And I am sure that dead reckoning
    longitude-errors in such a voyage, in 16-17th century vessels with 16-17th
    century technology, would have been greater still, by quite a long way.
    By the way, "a degree or two" in longitude might well correspond to only 38
    to 77 miles (not his 60 to 120), depending on at which end of a voyage
    between Cape Verde Is. and Lizard you choose to measure it. Indeed, that
    related to one of the "Certaine Errors" in navigation that Wright was doing
    his best to address; that mariners were assessing long ocean voyages as if
    they were simply plane sailing, without considering the spherical shape of
    the Earth.
    | My own readings indicate that navigators who had to depend on dead
    | managed much better results than this.  Again, this depends on the
    | circumstances; if crossing vast oceans without any correction then yes, a
    | degree or two could be a modest error in those circumstances.
    Peter Fogg does not tell us what "readings" he relies on to indicate that.
    | The voyaging we are considering here, from the Azores northeast towards
    | Britain, is more modest in scope, and a significant component involves a
    | change in latitude rather than longitude.
    We are talking about something like 2500 miles, assuming no adverse winds,
    and a month or so length of voyage if things go well, in a vessel which
    could only travel down, or across, the wind, with no ability to beat against
    it, which could double passage length and time if things go badly. In a
    vessel which averages 4  knots or so, and makes considerable leeway, which
    has to be estimated. Using dead-reckoning techniques before the invention of
    the ship-log, with a primitive compass when magnetic variation was not
    understood, through ocean currents about which nothing was known.
    It doesn't matter whether the ship's travel was North-South or East-West; in
    that month or more there is plenty of time to build up DR error, both
    across-track and along-track. To allow 120 miles error would correspond to
    getting the speed right within 5%, the average course within 3 degrees. I
    doubt if mariners of the era could even approach that standard.
    But if Peter thinks otherwise, perhaps he will provide his own estimate,
    taking the factors I have mentioned into consideration.
    contact George Huxtable at george@huxtable.u-net.com
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK..
    To post to this group, send email to NavList@fer3.com
    To unsubscribe, send email to NavList-unsubscribe@fer3.com

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