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On the globe, lines of constant longitude (“meridians”) extend from pole to pole, like the segment boundaries on a peeled orange.
Every meridian must cross the equator. Since the equator is a circle, we can divide it — like any circle — into 360 degrees, and the longitude \(\phi\) of a point is then the marked value of that division where its meridian meets the equator.
What that value is depends of course on where we begin to count — on where zero longitude is. For historical reasons, the meridian passing the old Royal Astronomical Observatory in Greenwich, England, is the one chosen as zero longitude. Located at the eastern edge of London, the British capital, the observatory is now a public museum and a brass band stretching across its yard marks the “prime meridian.” Tourists often get photographed as they straddle it–one foot in the eastern hemisphere of the Earth, the other in the western hemisphere.
A lines of longitude is also called a meridian, derived from the Latin meri, a variation of “medius” which denotes “middle,” and diem, meaning “day.” The word once meant “noon,” and times of the day before noon were known as “ante meridian,” while times after it were “post meridian.” Today's abbreviations a.m. and p.m. come from these terms, and the Sun at noon was said to be “passing meridian.” All points on the same line of longitude experienced noon (and any other hour) at the same time and were therefore said to be on the same “meridian line,” which became “meridian” for short.