Lines of Longitude are Great Circles East and West of the Prime Meridian

Prime Meridian Longitude
The Prime Meridian in Greenwich, England serves as the starting point for measuring degrees longitude around the world. Matt Rosenberg

Longitude is the angular distance of any point on Earth measured east or west of a point on Earth's surface.

Where Is Zero Degrees Longitude?

Unlike latitude there is no easy point of reference such as the equator to be designated as zero degrees in the longitude system. To avoid confusion, the world's nations have agreed that the Prime Meridian, which passes through the Royal Observatory in Greenwich, England, will serve as that reference point and be designated as zero degrees.

Because of this designation, longitude is measured in degrees west or east of the Prime Meridian. For example, 30°E, the line passing through eastern Africa, is an angular distance of 30° east of the Prime Meridian. 30°W, which is in the middle of the Atlantic Ocean, is an angular distance of 30° west of the Prime Meridian.

There are 180 degrees east of the Prime Meridian and coordinates are sometimes given without the designation of "E" or east. When this is used, a positive value represents coordinates east of the Prime Meridian. There are also 180 degrees west of the Prime Meridian and when "W" or west is omitted in a coordinate a negative value such as -30° represents coordinates west of the Prime Meridian. The 180° line is neither east nor west and approximates the International Date Line.

On a map (diagram), lines of longitude are the vertical lines running from the North Pole to the South Pole and are perpendicular to lines of latitude.

Every line of longitude also crosses the equator. Because longitude lines are not parallel, they are known as meridians. Like parallels, meridians name the specific line and indicate the distance east or west of a 0° line. Meridians converge at the poles and are farthest apart at the equator (about 69 miles (111 km) apart).

Development and History of Longitude

For centuries, mariners and explorers worked to determine their longitude in an effort to make navigation easier. Latitude was determined easily by observing the inclination of the sun or the position of known stars in the sky and calculating the angular distance from the horizon to them. Longitude could not be determined in this way because Earth's rotation constantly changes the position of stars and the sun.

The first person to offer a method for measuring longitude was the explorer Amerigo Vespucci. In the late 1400s, he began measuring and comparing the positions of the moon and Mars with their predicted positions over several nights at the same time (diagram). In his measurements, Vespucci calculated the angle between his location, the moon and Mars. By doing this, Vespucci got a rough estimate of longitude. This method did not become widely used however because it relied on a specific astronomical event. Observers also needed to know the specific time and measure the moon and Mars' positions on a stable viewing platform- both of which were difficult to do at sea.

In the early 1600s, a new idea to measure longitude was developed when Galileo determined that it could be measured with two clocks.

He said that any point on Earth took 24 hours to travel the full 360° rotation of Earth. He found that if you divide 360° by 24 hours, you find that a point on Earth travels 15° of longitude every hour. Therefore, with an accurate clock at sea, a comparison of two clocks would determine longitude. One clock would be at the home port and the other on the ship. The clock on the ship would need to be reset to local noon each day. The time difference would then indicate the longitudinal difference traveled as one hour represented a 15° change in longitude.

Shortly thereafter, there were several attempts to make a clock that could accurately tell time on the unstable deck of a ship. In 1728, clockmaker John Harrison began working on the problem and in 1760, he produced the first marine chronometer called Number 4.

In 1761, the chronometer was tested and determined to be accurate, officially making it possible to measure longitude on land and at sea.

Measuring Longitude Today

Today, longitude is more accurately measured with atomic clocks and satellites. The Earth is still divided equally into 360° of longitude with 180° being east of the Prime Meridian and 180° west. Longitudinal coordinates are divided into degrees, minutes and seconds with 60 minutes making up a degree and 60 seconds comprising a minute. For example, Beijing, China's longitude is 116°23'30"E. The 116° indicates that it lies near the 116th meridian while the minutes and seconds indicate just how close it is to that line. The "E" indicates that it is that distance east of the Prime Meridian. Although less common, longitude can also be written in decimal degrees. Beijing's location in this format is 116.391°.

In addition to the Prime Meridian, which is the 0° mark in today's longitudinal system, the International Date Line is also an important marker. It is the 180° meridian on the opposite side of the Earth and is where the eastern and western hemispheres meet. It also marks the place where each day officially begins. At the International Date Line, the west side of the line is always one day ahead of the east side, no matter what time of day it is when the line is crossed. This is because the Earth rotates east on its axis.

Longitude and Latitude

Lines of longitude or meridians are the vertical lines running from the South Pole to the North Pole. Lines of latitude or parallels are the horizontal lines running from the west to the east. The two cross each other at perpendicular angles and when combined as a set of coordinates they are extremely accurate in locating places on the globe. They are so accurate that they can locate cities and even buildings to within inches. For example, the Taj Mahal, located in Agra, India, has a coordinate set of 27°10'29"N, 78°2'32"E.

To view the longitude and latitude of other places, visit the collection of Locate Places Worldwide resources on this site.