Welcome to You Ask Andy

Michael Komoroski, age 11, of Pittsburgh, Pennsylvania, for his question:

Why is the Prime Meridian in England?

This tale goes back to a time when Britishers could claim Britannia Rules the Waves. Their little island and their far-flung colonies were guarded by a mighty navy that no other country cared to challenge. Fleets of their trading ships sailed the seven seas. All this seafaring required pilots who knew where they were at all times. This is how come the Prime Meridian was set up in England.

Lines of latitude and longitude were used to plot the surface of the globe, way back in ancient days. But our precise modern system dates back only a few centuries. The latitudes, as we know, are parallel circles, more or less evenly spaced between the equator and the two poles. These east west lines are crossed by evenly spaced lines of longitude that run north and south, from pole to pole. Each meridian is a half circle of longitude linking the two poles.

As we know, the planet Earth rotates on its axis toward the east, completing a day and night circle in about 24 hours. Because of this, the sun rises in the eastern sky and sets below the western horizon. its path is related to the passage of time, for it crosses 15 degrees of longitude every hour. Hence the half circle meridians are the keys to our timekeeping system.

The problem is solved by special timekeeping astronomers, who rely on a celestial system of latitudes and longitudes. For example, the celestial equator is directly above  the earth’s equator. There are celestial lines in the sky to match the poles and all the latitudes and longitudes.

A special observatory staffed by specially trained astronomers is needed to make use of this basic blueprint. When precisely charted, the passage of the sun can reveal the exact time. What’s more, the same basic system is related to where as well as to when. For the latitudes and longitudes chart every location on the face of the globe.

One of the earliest centers set up for the scientific study of when and where was the Royal Observatory of Greenwich, England. In 1710, with the most up to date equipment and staff of the times, it was the best of its kind. The seafarers of the world depended on the globe charters and timekeepers of Greenwich.

When it comes to where, there is no doubt at all about the north and south directions. The two poles are there to mark the spots. But there is no natural marker to divide the east and west. Hence it was necessary to invent one. Naturally the astronomers of Greenwich chose to place longitude 0 degrees right above their heads. This is the Prime Meridian, running directly north and south from pole to pole. In 1884, it was officially adopted by the civilized world.

From the Prime Meridian, 180 degrees are counted westward and 180 eastward. The east and west hemispheres meet at the International Date Line, which runs down the middle of the Pacific Ocean. Meantime, the Greenwich astronomers check the sun as it crosses their Prime Meridian. This precise moment of noon by Greenwich time is flashed around the world.

 

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