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    Re: How good were chronometers?
    From: Frank Reed CT
    Date: 2006 Mar 14, 21:33 EST

    Greg, you asked:
    "wasn’t the standard  practice of the day to do either:
    1.      Do a  simple noon sun shoot for Lat & Lon?
    2.      If  you couldn’t do a noon sun shoot for some  reason
    say a high cloud deck, then  you would try a equal alt.
    ( 2-4 hr before and  after) shoot. And interpolate local noon?
    3.      If  you were really good at the math, and had favorable conditions; a
    twilight  (morning or evening) sight of Polaris and another star, then work
    a full  spherical triangle?"
    A common "standard practice" was to do Noon Sun for  latitude and shoot a
    separate "time sight" altitude at 9am or 3pm, roughly, for  local time and thus
    longitude. At noon, you could set watches (average pocket  watches, not
    chronometers, of course) to 12:00 but this was not an accurate  enough measure of
    local time for longitude. Equal altitude sights can get you  the time of local
    noon rather accurately, but you have to hope that you get  clear weather
    symmetrically on both sides of noon. You also have to correct for  the motion of the
    vessel, as in any running fix. Stars, including Polaris, were  rarely used in
    celestial navigation before the 20th century except occasionally  for lunar
    distance sights.
    So what's a "time sight"? You can look at some  examples worked up in the
    1896 "Navigation Workbook" of the Charles W. Morgan on  Mystic Seaport's library
    web site. You can also find them in Bowditch. The name  "longitude by
    chronometer" in older navigation works usually refers to plain  time sights. In short,
    a time sight turns a sextant into a very accurate  sundial. By measuring the
    Sun's altitude and working a spherical triangle  calculation, with the Sun's
    declination and your dead reckoning latitude as  additional inputs, you can
    calculate the Sun's Local Hour Angle. Converted to  time units, the Sun's LHA
    *is* the Local Apparent Time --sundial time. After  applying the equation of
    time, comparing local time with chronometer time yields  the longitude directly.
    The sight reduction process takes about five  minutes.
    To get a Sumner line, you simply work the time sight again with  a different
    assumption for the dead reckoning latitude.
    [Note: I  mentioned shooting time sights at 9am, 3pm. I'm not saying that
    this was  recommended technique, only that it was a common technique (I don't
    know why).  Recommended technique is to take the time sights when the Sun is as
    close to  true East or true West as possible, so long as it's not less than 5
    degrees in  altitude]
    42.0N 87.7W, or 41.4N  72.1W.

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