On this day in 1769, an eclipse of one of Jupiter’s four moons took place.
Having recently arrived at Tahiti, Captain Cook and the Endeavour’s astronomer Charles Green went ashore to witness the event.
In the days before good chronometers were available, longitude measurements were based on the transit of the moon, or the positions of the moons of Jupiter.
Both Cook and Green were well versed in these techniques, including the one put forward in 1610 by Galileo Galilei.
As the first man to turn a telescope to the skies, he discovered the four moons orbiting Jupiter. He named them Cosimo, Francesco, Carlo and Lorenzo after the sons of Cosimo II de’ Medici, the Grand Duke of Tuscany, in the hope of securing his patronage.
However, the German astronomer Simon Marius also claimed to have discovered the moons around the same time, and gave them the mythical names by which we know them today – Io, Europa, Ganymede and Callisto.
These four moons experienced eclipses so predictably that Galileo claimed one could set a watch by them. For Cook and Green the April 17 eclipse was valuable preparation for the upcoming transit of Venus. In the event, the weather was unfavourable.
“In the after noon we set up one of the Ships Tents a Shore and Mr Green and myself stay’d a Shore the night to Observe an Eclipse of Jupiter’s fi[r]st Setilite which we was hinder’d from seeing by clowds,” Cook wrote in his journal.
It was a stark reminder of the challenges they could yet face in achieving their aim of observing the Venus transit on June 3.
For more on the transit of Venus and the longitude problem click here.
The above photo comes via David Wiley on Flickr.