Finding longitude
Longitude is a matter of time: the Earth turns 15 degrees an hour, so the difference between two local clocks gives the distance east or west.
Longitude tells you how far east or west you are. Because the Earth turns once in 24 hours, every 15 degrees of longitude is one hour of local time. If it is noon where you stand and 2 p.m. at a place whose time you know, you are 30 degrees west of it.
The hard part was knowing the time at the faraway place. Before reliable sea clocks, astronomers used events in the sky that everyone sees at the same moment, such as the eclipses of Jupiter's moon Io. In 1671, timings of the same eclipses on the Danish island of Hven and in Paris differed by about 42 minutes, which puts Hven roughly 10.5 degrees east of Paris.
A common misconception is that latitude and longitude are equally easy to find. Latitude comes straight from the height of the Sun or the Pole Star; longitude needs a clock, which is why it took centuries to solve at sea.
