250 years ago, ships often became lost in vast oceans.
Sailors could tell how far north or south they were by the Sun's position in the sky.
But they had no way of telling how far east or west they'd sailed.
For over 1000 years, cartographers had been using grid-lines, known as latitude and longitude, to accurately map the world.
Lines of Latitude
Lines of latitude circle the Earth in parallel, horizontally.
The equator is the most famous line of latitude.
Because the mid-day Sun is highest at the equator, latitude can be calculated from the angle between the Sun and the horizon.
Latitude = angle of Sun to horizon
Lines of Longitude
But calculating longitude was different.
Longitude is depicted by the vertical lines running from pole to pole.
1 hour time difference =
15 degrees travelled east or west
For every 15° shift east or west the Sun will rise an hour later or earlier.
To know your longitude, you must know the time in two places at once.
For example, if you know that back at port it's 2pm, but you have used the Sun to calculate it's high noon where you are, you must be 30° west of port.
Sailors could use the Sun to tell the local time, and they tried to track what time it was back at port with pendulum clocks.
But these were hopelessly inaccurate, because they were affected by the swell of the sea.
John Harrison, 1693-1776
Carpenter and clockmaker
It was a poorly educated carpenter's son, called John Harrison, who'd solve the longitude problem.
Harrison was obsessed with clocks, and quickly grasped that a clock that relied on mechanics, not gravity, was needed aboard ships.
It took Harrison 31 years to build his exceptionally accurate portable clock, called a chronometer.
It revolutionised naval navigation.
Nowadays, these precision timekeepers are now more often used to set watches.
Ships today are more likely to rely on the GPS, or Global Positioning System.
GPS uses atomic clocks, which measure time using high frequency vibrations – to give us the latitude and longitude of anywhere on Earth.
But still, the secret of knowing your position in the world, is to know the time at two locations.