Longitude represents the geographic coordinate that specifies the east-west position of a point on the Earth’s surface. It is measured in degrees east or west of the Prime Meridian in Greenwich, England. Unlike latitude, which was easy to find using the stars, calculating longitude historically required highly accurate clocks (chronometers) to compare local time with Greenwich time. One specific example involves a sailor checking their GPS and seeing they are at 70 degrees West longitude, indicating they are in the Western Hemisphere. Combined with latitude, it allows for a precise ‘fix’ on a chart. Understanding longitude is essential for any long-distance navigator traveling across the world’s oceans. It defines the time zones and the global coordinate system.