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    Re: Bligh's noon by chronometer
    From: George Huxtable
    Date: 2010 Jun 1, 10:18 +0100

    Gary wrote-
    " I've never understood the slavish devotion to catching the sun at the
    exact highest point. If instead you work the noon sight as a normal LOP
    sight using the normal tables (HO 214 etc.) you have a four minute period
    when you work the sight with an LHA of zero and an AP within 30 NM of the
    DR and this method produces an LOP of the normally expected accuracy."
    Indeed, the available window, for observing the Sun near noon, without
    needing to correct for the time difference from noon, is generally much
    wider than Gary's four minutes. But say the Sun wasn't visible at, or near,
    noon, but only deigned to pop out an hour or so later. That was when the
    ex-meridian tables were needed.
    Gary could work such a sight, when the Sun appeared at any time before or
    after noon, using sight reduction tables, and drawing the resultant LOP on
    a plotting sheet. And then, that could be crossed with another LOP at a
    different time, to provide a fix.
    But he is looking at the matter from a modern viewpoint, and needs to think
    himself back into the late 18th century. The concept of a position-line
    hadn't, then, been accepted. Instead, latitude and longitude were thought
    of as separate quantities, of different types, and a navigator needed to
    know them independently.
    There was some justification for this. Latitudes could be measured at sea,
    then, just about as precisely as they can, by us, now. Until a few years
    before Bligh's era, before Greenwich time became available, longitudes
    could be estimated only by DR, a form of unrefined guesswork, with enormous
    errors. If they had thought about it in terms of an error-ellipse, then, in
    mid-passage that ellipse might have been a few minutes tall (in latitude)
    and many degrees broad (in longitude).
    Even after knowlege of time became possible, first from lunars and then
    from a chronometer, that knowledge was imperfect; especially after a long
    voyage with an early timekeeper (the situation Bligh was in).
    Error-ellipses would still be very wide compared with their height. To
    Gary, and the rest of us, today, knowledge of precise time comes easy. We
    can thus determine longitudes just as precisely as latitudes, and a
    position-line then becomes a natural concept. But until the days of Sumner,
    half a century after Bligh, when chronometers had become cheap, accurate,
    and dependable (and carried in sets of three), latitude and longitude were
    thought of quite differently.
    What Bligh wanted, then, was a latitude measurement he could put on his
    chart. A Sun position line would have meant nothing to him.
    contact George Huxtable, at  george@hux.me.uk
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

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