Longitude

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.

Although lines of longitude may be imaginary, they are very important to navigators; Lewis Carroll joked about this in his long nonsense poem The Hunting of the Snark:

"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 replyv

"They are merely conventional signs!

Accordingly, the Snark uses a map which is "a perfect and absolute blank."