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Latitude and Longitude
Factfile |
 Latitude and Longitude |
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Lines of Longitude or 'Meridians' |
Since the equator is a circle,
we can divide it--like any circle--into 360 degrees, and
the longitude f of a point is then the marked value of
that division where its meridian meets the equator.
In the diagram below, for example the Point P is at 40 degrees longitude. |
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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 line of longitude is also called a meridian, derived from the Latin, from 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.
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