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    Re: [searoom] POB and Jupiter's moons' eclipses
    From: Carl Herzog
    Date: 2004 Feb 13, 16:21 -0500

    The specific observations made were eclipses, not occultations or transits.
    One was to observe the "times of immersions (signifying the instants of
    disappearance of a satellite on entering the shadow of Jupiter) and
    emersions (signifying the reappearances of satellite on emerging from
    Jupiter's shadow)." [History of Nautical Astronomy, Charles Cotter]
    It was first proposed by John Flamsteed in the late 1600s and the first
    calculated distances appeared in the British Nautical Almanac of 1765. (I
    don't know what year these calculations were discontinued.)
    As John Forrester indicates, there were numerous impracticalities associated
    with this technique. Compounding those he mentioned are the need to discern
    the semi-diameter of the lunar body in order to accurately gauge the
    beginning of the eclipse and the need to determine the errors caused by
    atmospheric refraction.
    Despite these shortcomings, astronomers viewed this as a better method for
    determining longitude than the use of our own lunar eclipses -- based on
    difficulties prediction our moon's orbit.
    As in all astronomical methods, you compare the local time of your sight
    with the time in the almanac. Local time was calculated by observation of
    the sun and a little trig -- determing the angle between the sun's azimuth
    and your meridian provides the time before or after local apparent noon.
    Again, it is important to point out that all these efforts were being
    considered many years before POB's Aubrey would have sailed. By the time
    Jack went to sea, lunar distances were the preferred method. This involved
    measuring the angular distance between the moon and another celestial
    body -- the sun or a star. Predicted measurements between the moon and the
    common navigational stars appeared in the nautical almanac.
    Carl Herzog

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