Longitude

Now imagine travelling from the North Pole to the South Pole by the shortest possible line. Look at the globe: instead of passing through Europe and Africa, you could just as well pass through Asia, and the distance would be the same.

These lines are called meridians of longitude. They are all half-circles running from one pole to the other, and all the same size. Compare latitude: parallels are full circles of different sizes; meridians are half-circles, all alike.

Longitude is linked to time. Because the Earth spins, it is morning in some places while it is midday, evening or night in others: breakfast time in one country, lunchtime in another, and people fast asleep in a third. So by measuring a place's longitude, we also measure the time there.

To measure longitude we need a starting line, called the Prime Meridian (or Greenwich Meridian). In 1884 some nations decided that the meridian through Greenwich, a part of London in England, would be the world standard. It is marked 0° longitude. Notice the word decided: the Equator is fixed by the Earth itself, halfway between the poles, but the Prime Meridian is a choice.

Longitude is the distance from the Prime Meridian, measured along the Equator. Going east or west it grows from 0° to 180°, with the letter E or W added. New York's longitude is 74°W, Delhi's is 77°E and Tokyo's is 140°E. And 180°W and 180°E are the same line, written simply 180°.

Latitude and longitude together are the two coordinates of a place, and with them you can find any place on Earth. For example, Delhi lies at 29°N latitude and 77°E longitude. All the lines together make a grid over the globe, so they are also called grid lines. The Prime Meridian splits the world into the Western and Eastern Hemispheres; the Equator splits it into the Northern and Southern Hemispheres.

A piece of history: the Greenwich Meridian was not the first prime meridian. Many centuries before Europe, India had its own, called the madhya rekhā ('middle line'). It passed through the city of Ujjayinī (today Ujjain), a famous centre of astronomy for many centuries. The great astronomer Varāhamihira lived and worked there about 1,500 years ago. Indian astronomers understood latitude and longitude, including the need for a zero meridian, and the Ujjayinī meridian was the reference for calculations in all Indian astronomical texts. Some cities said to lie on it are a little off it, because measuring longitude needed accurate timekeeping, which was not as precise then as it is today.