What Is Longitude and How Do We Measure It?

Longitude measures how far a place is east or west on Earth. People measure it using lines called meridians that run from the North Pole to the South Pole. The line at 0 degrees is called the prime meridian. In 1884, leaders chose a path through Greenwich, England, for this line because many maps already used it. Today, scientists use satellites to measure longitude more accurately. The line on the opposite side of Earth is 180 degrees longitude, which forms the idea for the international date line.

Latitude lines always stay the same distance apart, but longitude lines do not. Longitude lines are widest at the equator and meet at the poles. Finding latitude was easy for thousands of years, but longitude was much harder to find. At sea, sailors needed accurate clocks to calculate their location. They compared their local time with the time in Greenwich to find their longitude.

Normal clocks did not work well on ships because of sea movement and weather. In the 18th century, people made special clocks that worked well at sea. Today, very exact atomic clocks travel on GPS satellites in space. Your phone can use these satellite signals to find your position. Scientists can even measure a location to a few centimeters using ground stations.

Why do you think it is important for ships and travelers to know their exact location?