What is the difference between latitude and longitude?

Latitude measures how far north or south a place sits, from 0 degrees at the equator to 90 degrees at each pole. Its lines run parallel to the equator, which is why they are called parallels, and they never converge. Longitude measures east or west position, from 0 degrees at the Prime Meridian through Greenwich to 180 degrees, and its lines, called meridians, all converge at the poles, so a degree of longitude represents about 69 miles at the equator and shrinks to nothing at the poles. The common memory aid is that latitude lines are flat like the rungs of a ladder.

The historical asymmetry between them is worth knowing. Latitude was determinable at sea from antiquity by measuring the angle of the sun at noon or the elevation of Polaris. Longitude required knowing the time at a reference location simultaneously with local time, which meant carrying an accurate clock through months of heat, humidity, and ship motion, a problem no pendulum clock could survive. Britain's Longitude Act of 1714 offered an enormous prize, ultimately claimed largely by clockmaker John Harrison, whose marine chronometers solved it after decades of work and institutional resistance. Until then, ships routinely could not tell how far east or west they were, and shipwrecks from the resulting errors killed thousands.