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    Whoops, wrong list.. Re: [searoom] POB and Jupiter's moons' eclipses
    From: Carl Herzog
    Date: 2004 Feb 13, 16:34 -0500

    ----- Original Message -----
    From: "CarlZog" 
    Sent: Friday, February 13, 2004 4:21 PM
    Subject: Re: [searoom] POB and Jupiter's moons' eclipses
    > The specific observations made were eclipses, not occultations or
    > 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
    > with this technique. Compounding those he mentioned are the need to
    > 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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