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November 2003 Feature: November 8th Total Eclipse of the Moon



On the night of November 8-9, a total eclipse of the Moon darkens the skies of North and South America, Europe and Africa. Although European and African nations won't witness the beginning of the eclipse till around midnight or even later, we in the eastern United States see the Moon moving into the Earth's dark shadow (umbra) at early evening. In our Eastern Time Zone, the eclipse starts at 6:32, with totality lasting from 8:06 to 8:31, and a partial eclipse remaining till 10:05 p.m.*

An Historic Eclipse

This eclipse serves to remind us of a special total lunar eclipse that took place on this same date 372 years ago: November 8-9, 1631. Like the upcoming eclipse, the 1631 production was visible in its entirety from both England and eastern North America, prompting an adventurous Old World professor and a scholarly New World explorer to try their hand at eclipse wizardry. Back then, lunar eclipses offered the only viable means of measuring across the vastness of the Atlantic Ocean, and they wanted to use this eclipse to figure the distance from London, England to Charlton Island in James Bay, Canada.

In 1631, it was exactly one century before the invention of the double-mirror quadrant (the forerunner of the modern marine sextant) and more than a century before the invention of a sea-worthy chronometer (clock). Accordingly, finding longitude, or east-west distance at sea in the seventeenth century was close to impossible, whereas measuring the difference in longitude between Old and New World stations on land was simply difficult -- but at least feasible.

Henry Gellibrand, professor of astronomy at Gresham College in London, England, and his partner, Captain Thomas James, attempted the latter. Given that the eclipse would be viewed simultaneously from both London and Charlton Island, they counted on the time difference of the eclipse to reveal their difference in longitude. So on the night of November 8-9, 1631, the professor and the captain carefully recorded the local times of the eclipse at their respective stations; and sure enough, when they compared notes at a later date, the dynamic duo managed to conjure up the longitudinal distance between the two points.

They found the local time of the eclipse at Charlton Island to be 5.3 hours earlier than in London, England. Since an hour earlier corresponds to a difference of 15 degrees of longitude westward, that places Charlton Island 79.5 degrees to the west of London (15 x 5.3 = 79.5). Quite impressive, considering the modern value is something like 79.3 degrees!

Enjoy the upcoming total lunar eclipse this November 8, a most fitting testimonial to the memory of Professor Henry Gellibrand, Captain Thomas James and the eclipse of November 8-9, 1631!


copyright 2003 by Bruce McClure


* click here to see a map of this eclipse, courtesy of Eclipse Predictions by Fred Espenak, NASA/GSFC

Resources:
Cambridge Illustrated History of Astronomy by Michael Hoskin
Emily Winterburn, Curator of Astronomy, Royal Observatory, England
The Strange and Dangerous Voyage of Captain Thomas James by Captain James (1633)

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