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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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