Lines of longitude or meridians are imaginary lines that run north and south across the earth. They are used to measure east-west location, relative to a "prime meridian" which by definition is 0°. While in the past many meridians have been used, since 1884 the Greenwich Meridian has become the universal standard.
Longitude is an angular measurement and is given in degrees, minutes, and seconds, represented by the symbols °, ', and " respectively. The longitude of the NOAA weather station in New Brunswick, New Jersey, for example, is 74°26'W; that is to say, seventy-four degrees, twenty-six minutes West of the Greenwich meridian.
At the Equator, the circumference of the Earth is about 25,000 miles; so one degree of longitude is about 25000 / 360 = 69.4 miles, and one minute is 69.4/60 = 1.15 miles. Originally, a nautical mile was intended to be one minute of longitude at the equator. (The nautical mile is now defined to be exactly 1852 meters, so a minute of longitude is actually 1.0018 nautical miles). A second of longitude is only about a hundred feet; before GPS, it was difficult to quickly measure longitude to an accuracy of a second.
In 1995, a book entitled simply Longitude, by Dava Sobel, made the nonfiction bestseller list. It tells the story of John Harrison's development, in the 1700s, of the first marine chronometer accurate enough to determine longitude at sea.
- "What's the good of Mercator's North Poles and Equators,
- Tropics, Zones, and Meridian Lines?"
- So the Bellman would cry: and the crew would reply,
- "They are merely conventional signs!
Accordingly, the Snark uses a map which is "a perfect and absolute blank."