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    Another reply to John Mc Keel
    From: Jan Kalivoda
    Date: 2002 Dec 31, 17:36 +0100

    Another cause of starting the day at noon among astronomers was probably the
    unreliability of pendulum clocks at the end of 17th century and later.
    Astronomers used to ascertain the error of their clock every day then, if
    possible. The easiest way for it was the moment of Sun's upper meridian
    transit. Having determined the clock error at noon, astronomers were ready
    for night observations. And from 1679, when the first astronomical and
    nautical yearbook,
    "Connaissance des temps", emerged, the noon became the main point of the day
    in ephemerids for a long time . Up to the 20th century, some data were given
    for the apparent noon, not for the mean one or midnight in nautical and
    astronomical almanacs.
    When the change in beginning the astronomical day arrived in 1925, some mess
    sprung up in the nautical astronomy. The German formed another term beside
    the "hour angle" - so called "time angle" reckoned from the lower part of
    meridian from 0 to 24 hours, so that they could cope with the new begin of
    astronomical day. It was quite logical and quite abstruse. The British, in
    their well-known reverence for men of sea, have then introduced two new
    quantities into the Nautical Almanac instead of former "Right Ascension of
    the Mean Sun at Noon" and "Equation of Time" - nameless values R and E, by
    help the seadogs could ignore the shift of the day's outset in their sight
    reductions and continue as previously. That arrangement survived up to the
    WW II, when nautical and air almanacs gradually began to tabulate the hour
    angles of celestial bodies for each hour / ten minutes directly.
    The last remnant of the old start of the astronomical day at noon is the era
    of the "Julian days", used for counting long intervals into the past and
    future in astronomy. These Julian days begin at noon up to now, because
    trying to suppress or shift half a day in their reckoning would destroy the
    order of heaven.
    Jan Kalivoda

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