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    Re: Mid XIX century Nav
    From: Frank Reed CT
    Date: 2005 Nov 22, 01:29 EST

    George H, you wrote:
    "A modern Times Atlas  gives the longitude of Nagasaki as 129 deg 52 E, which
    is exactly the same  as  Raper (in 1864) gives for "Nangasaki". What is
    remarkable is the  agreement, with a minute, between that figure and the
    long. quoted by Frank  Reed from Kruzenschtern of 230 deg 07W, as early as
    I should  emphasize again that the longitude on the chart is "maybe" 230 07W.
    It's right  at the limit of readability on the NYPL web site, and I may be
    influenced by the  known longitude of the place.
    And you wrote:
    "Frank made the  interesting suggestion that a lunar occultation might have
    been employed to  get the "timing" error of the chronometer, and indeed, so
    it could.  "
    I agree with you about the relative impracticality of using  occultations,
    but actually, I mentioned "moon culminations" which were very  popular for
    land-based determinations of longitude in the first half of the 19th  century.
    These are apparently more practical than occultations.
    And you  wrote:
    "The answer lies in the stars. If the navigator goes ashore, and sets  up a
    post with a clear view of the Southern sky behind it, and some sort  of
    peep-hole to position his eye well in front of it, he can time  low-altitude
    stars as they disappear, instantaneously, behind that post. The  interval
    between passages of the same star is exactly one sidereal day, which  is just
    3 minutes 55.91 seconds (and umpteen decimal places) short of a day  by GMT."
    This is a good method for measuring a sidereal day if you have  few
    instruments (or a hundred years earlier), but I think in the mid-19th  century the
    rating was done much more simply. You do time sights a few days  apart with an
    artificial horizon (if possible). If the chronometer is five  seconds slow on
    Monday at 4pm and seven and a half seconds slow on Friday at  4pm, then the rate
    is 0.6 seconds per day. Then, let's say, three weeks later,  at your next port
    of call with a well-established longitude, you would expect to  find that the
    chronometer is 20 seconds slow if the rate has been constant. If  not, you've
    got a new rate determination, or trouble.
    42.0N  87.7W, or 41.4N 72.1W.

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