Q: A friend of mine was up in England recently, and told me he found a spot where he could have one foot in the eastern hemisphere and one foot in the western hemisphere. What’s up with that?

A: Ah, your buddy must have been in Greenwich, through which the prime meridian runs. The prime meridian is an imaginary line that runs from the North Pole to the South Pole.

It marks zero degrees longitude, and is the line from which all other lines of longitude are measured. This includes the line that runs 180 degrees away from Greenwich, also known as the International Date Line.

According to the Web site, the prime meridian runs through “the primary transit,” instrument — or the main telescope at the Royal Observatory in Greenwich, a London borough southeast of the city center.

Similarly, the equator marks zero degrees latitude, and divides the Northern Hemisphere from the Southern Hemisphere. It is halfway between the North and South poles.

The earliest maps have the equator marked on them, but until the late 19th century, as many as 14 different locations were being identified on various maps as zero degrees longitude. The International Meridian Conference of 1884 decreed that the imaginary line drawn through Greenwich would be the prime meridian forevermore.

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