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