Latitude and longitude

One degree of latitude covers about 111 kilometres anywhere on Earth, but a degree of longitude shrinks to nothing at the poles.

Latitude: how far north or south

Any place on Earth can be named with two numbers. The first, latitude, tells you how far north or south of the equator you are. Lines of latitude, also called parallels, run east and west around the globe. The equator is 0 degrees and divides the Earth into the Northern and Southern Hemispheres. Latitude rises as you go towards either pole, reaching 90 degrees north at the North Pole and 90 degrees south at the South Pole. The Arctic and Antarctic Circles are at roughly 66 degrees north and south.

Latitude is an angle, not a distance. Draw a line from the centre of the Earth to your position and another from the centre to the equator on the same meridian as you; the angle between them is your latitude. Because the parallels are evenly spaced, each degree of latitude covers about 111 kilometres on the ground. A degree is split into 60 minutes and each minute into 60 seconds, so a second of latitude is only about 30.8 metres.

Longitude: how far east or west

The second number, longitude, tells you how far east or west you are. Lines of longitude, called meridians, run from pole to pole. Unlike the equator, which nature puts halfway between the poles, the starting line for longitude is a choice. In 1884, delegates from 25 nations met in Washington, D.C., at the International Meridian Conference and chose the meridian through the telescope at the Royal Observatory in Greenwich, England. The vote was 22 to 1, with San Domingo against and France and Brazil abstaining. Many sea charts already used Greenwich, which made the choice easier.

Longitude runs from 0 degrees at Greenwich to 180 degrees east and 180 degrees west, which meet on the opposite side of the world at the antimeridian. Meridians are farthest apart at the equator, where a degree of longitude is about 111 kilometres, the same as a degree of latitude. They draw together towards the poles: by 60 degrees north or south a degree of longitude is down to about 56 kilometres, and at the poles it is zero. The rule is the cosine of the latitude, so at 60 degrees, where the cosine is 0.5, the distance is half.

Reading a position

A position is written latitude first, then longitude. The World Factbook gives Seoul as 37 33 N, 126 59 E, which means 37 degrees 33 minutes north and 126 degrees 59 minutes east. Many apps write the same place in decimal degrees. Thirty-three minutes is 33 ÷ 60 = 0.55 of a degree and 59 minutes is about 0.98, so Seoul is about 37.55° N, 126.98° E. In decimal form, south latitudes and west longitudes are often written as negative numbers, so a place in Brazil at 15 degrees south and 48 degrees west becomes −15, −48.