Longitude measures how far east or west you are on Earth's surface. Imagine the planet wrapped in invisible lines running from the North Pole to the South Pole, like the segments of an orange. These are lines of longitude, and they tell you your position in the east-west direction.

The starting point, called the Prime Meridian, runs through Greenwich, England, and is marked as 0 degrees longitude. From there, longitude is measured in degrees up to 180° east or west. New York City sits at about 74° west longitude, while Tokyo is at about 140° east longitude.

Longitude became crucial for navigation once sailors ventured far from shore. Without knowing their longitude, ships would sometimes miss their destination by hundreds of miles, or worse, crash into unexpected coastlines. The problem was so serious that in 1714, the British government offered a huge prize to anyone who could figure out how to calculate longitude accurately at sea. A clockmaker named John Harrison eventually won by inventing an extremely precise timepiece called a marine chronometer.

Today, GPS satellites calculate your exact longitude (and latitude) instantly, but for centuries, determining longitude was one of the hardest scientific challenges people faced.