Galileo discovered the four largest moons of Jupiter (Europa, Io, Callisto and Ganymede). He devised a method to determine one's longitude on Earth by timing the disappearance of these moons behind the planet.
Unfortunately, this was impossible to do on a moving ship and so was only really useful on land. But, apparently the method was used by Cassini and Jean Picard to produce a new map of France.
|