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    Re: Determining Actual Time of Meridian Passages Before Computers
    From: Alexandre Eremenko
    Date: 2007 Jan 15, 11:16 -0500

    On Sat, 13 Jan 2007, Greg R. wrote:
    > but how was it done in years past?
    > My best first-guess for a method of doing it
    Your guess is correct, but of course one has to distinguish
    between observations from a firm platform
    (land observatory) and from the sea.
    (On long voyages, especially those voyages where
    the purpose was at least partially scientific,
    seamen also used land observations. They carried
    potrable observatories, with meridian circles and
    astronomical clocks which could be fixed on the shore).
    Even at the time chronometers were available, they
    still made these shore observations to check chronometers).
    The instrument used on land was called Meridian circle.
    (But one can use many other instruments like theodolites)
    It would be fixed in the direction of the meridian
    on a stone platform with a stable foundation.
    It will have a micrometer in its field of view.
    For the time, they used "astronomical chock",
    something which looks like a clock which is called
    "grandfather clock" in the US:-)
    That is a pendulum clock fixed on a stable foundation.
    Such clocks reached very high accuracy long before
    sea-going chronometers were invented.
    (They were also used to test chronometers after chronometers
    were invented).
    There is a good discussion of these instruments
    (and math involved) in Chauvenet.
    As I already wrote on this list, Chauvenet is back
    to print again.
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