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What is the Prime Meridian? - Definition, Facts & Location

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  • 0:01 Defining the Prime Meridian
  • 2:09 History of the Prime Meridian
  • 3:10 Importance of the…
  • 4:43 Lesson Summary
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Lesson Transcript
Instructor: Linda Fye
Learn about the Prime Meridian and how, when, and why it was established. This lesson will help you to understand its importance to Earth's geographic grid and its importance to both location and time.

Defining the Prime Meridian

Look at this globe. On most globes, all the way around the sphere, you'll notice there are light lines drawn vertically as well as horizontally. These lines make up the Geographic Grid, a grid system that was created for Earth so that a location on the planet could be more easily described.

The grid is made up of lines of latitude, or parallels, and lines of longitude, or meridians. Lines of latitude run horizontally around the globe and define how far north or south a location is from the Equator; the equator is an imaginary line that is equidistant from both the North and South Poles that divides the Earth into northern and southern hemispheres.

The geographic grid of Earth
The geographic grid of Earth

The other lines - the ones that run vertically on the globe - are called lines of longitude. Lines of Longitude define how far east or west a location is from the Prime Meridian, the focus of this lesson. The Prime Meridian is an imaginary line that, similar to the Equator, divides the earth into eastern and western hemispheres. It is sometimes referred to as the Greenwich Meridian.

All lines of latitude and longitude are measured in degrees. As the base line for parallels, the Equator is considered to be 0 degrees latitude. Going north or south, latitude lines increase in degrees until we get to 90 degrees north at the North Pole and 90 degrees south at the South Pole.

Lines of longitude are slightly different. Lines of longitude run vertically around the planet from 0 to 180 degrees. They are not parallel. Instead, they cross each other at the poles and are farthest apart at the Equator. The Prime Meridian, as it passes through Greenwich, England, is considered 0 degrees longitude.

Because both the Equator and the Prime Meridian are imaginary lines, they were both established by mankind at some point in human history. The Equator was easy to decide because it's exactly between the North and South Poles. When you spin your globe, however, you'll notice that there's no obvious middle to an endless circle. So how, when, and why was the Prime Meridian established?

Prime Meridian
Prime Meridian

History of the Prime Meridian

Let's take a look at the history of of the Prime Meridian, beginning in the 1800's. At the time, railroads had become a major mode of transportation in the United States and Canada. In 1883, the two nations established a standard time system for North America to make railroad travel more dependable. It was rather successful and in October 1884, the International Meridian Conference in Washington DC was held so that a global standardized time system could be established.

Part of the challenge in establishing a global time system was deciding where it's starting point would be. While the Equator is obviously the starting point for lines of latitude, the Prime Meridian's location had to be chosen. At the time of the conference, each country had there own line of longitude that they considered to be the meridian line, or starting line from which longitude was measured. After lots of debate, the meridian that passes through the Royal Observatory in Greenwich, England, was chosen. The reason was simple and practical: over 43 of the world's shipping lines used it as a navigational base.

Laser pointing North along the Prime Meridian at the Royal Observatory in Greenwich, England
Laser pointing North along the Prime Meridian at the Royal Observatory in Greenwich, England

Importance of the Prime Meridian

The geographical grid allows us to accurately describe locations on Earth in a universal way. For example, if there was an old sunken ship with a treasure on it in the Atlantic Ocean, it would be very difficult to locate it by description alone. The geographical grid solves that problem. If you were told that it was located at 24 degrees north latitude and 41 degrees west longitude (24°N, 41°W), it would be much easier to locate.

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