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As we have already read, the lines extending around the Earth horizontally are called lines of latitude. They measure how far north or south an object is on the Earth. The lines running vertically around the Earth are called longitude. These lines are called meridians, and measure how far east or west an object is.

As with latitude, longitude is measured as an angular distance. With latitude, the angular distance is measured from the equator. This is a natural place to begin. However, with longitude there is no natural beginning point. One of these lines must be designated as a prime meridian, or as the 0 ° mark, but which one? For many years, each country had its own prime meridian. France used a meridian passing through Paris, while England used a meridian passing through Greenwich, England. Either one worked just as well, as long as you were only communicating with people from the same country. But what happens as technologies improve, and travelers begin interacting with people from many different lands? If each country is using a different prime meridian to mark 0 ° longitude, then it would be impossible to give someone from another country a location, and have them be able to accurately find it.


Over time, more and more travelers began to recognize the meridian passing through Greenwich, England, as the Prime Meridian. In 1884, a group of scientists, navigators, and businessmen made Greenwich, England, the official worldwide Prime Meridian.

The further away from the Prime Meridian that one travels the higher their longitude becomes, until they reach 180 ° longitude. If an individual is in the Eastern Hemisphere, their longitude is measured in degrees east. If they are in the Western Hemisphere, their longitude is measured in degrees west.