NavList:

I'm reading "Chasing Venus - The Race to Measure the Heavens".(1) It's essentially about astronomers dispatched from various countries to disperse locations worldwide to measure the June 6th 1761 transit of Venus. Their goal was to use timings of Venus' ingress, transit and egress across the face of the Sun (a rare event) to establish a parallax value that, in turn, could determine the Earth-Sun distance and begin to size the solar system. Dispersed viewing locations were required in order to witness the timing of each phase of transit and to increase the accuracy of the parallax value they would eventually compute. Many countries justified the expense of their expeditions as potential contributions to navigation at sea.

Nevil Maskelyne, a future Astronomer Royal and important contributor to navigation, was involved and set up his observation post on the island of St. Helena.

Some astronomers, such Swede Anders Planman in Kajana (eastern Finland) and Frenchman Chappe d'Auteroche in Tobolsk (Siberia, Russia) planned to calibrate their clocks and determine the longitude of their observing sites by observing a lunar eclipse that preceded the transit.

My question:
Although not an every day occurance, how can a lunar eclipse be used to determine longitude?
Eclipses tend to slow and gradually evolving events leaving in doubt exactly when the event climaxed. It's not unlike determining the exact moment of LAN.


The closest I found in NavList archives was this reference but it still does not say how.

Re: Lunar eclipses and other things
From: Lisa Fiene  Date: 2004 Oct 27, 09:55 +1000
http://fer3.com/arc/m2.aspx/Lunar-eclipses-other-things-LisaFiene-oct-2004-w19248


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(1) Andrea Wulf, "Chasing Venus, the Race to Measure the Heavens", Vintage Books Random House, 2012.