In 1714 the British Parliament passed the Longitude Act, offering a prize of up to £20,000 for a practical way to find a ship's longitude at sea. Sailors could already work out how far north or south they were. How far east or west was the hard part, and getting it wrong could put a ship on the rocks. One answer was a clock: the English clockmaker John Harrison spent decades building sea clocks accurate enough to do the job. The grid he was helping sailors use is the same one your phone uses today.
Latitude: north and south
Latitude measures how far a place is north or south of the equator, in degrees from 0° at the equator to 90° at each pole. Lines of latitude are called parallels because they run east–west and never meet.
A degree of latitude is about 111 km (69 miles) almost everywhere. A few parallels are worth knowing by name:
| Line | Latitude (approx.) |
|---|---|
| Arctic Circle | 66.6°N |
| Tropic of Cancer | 23.4°N |
| Equator | 0° |
| Tropic of Capricorn | 23.4°S |
| Antarctic Circle | 66.6°S |
The tropics mark the furthest north and south the Sun is ever directly overhead. Beyond the polar circles there's at least one day a year when the Sun doesn't set. These lines drift very slowly as the tilt of the Earth's axis changes, so the figures are rounded.
Longitude: east and west
Longitude measures how far a place is east or west of the prime meridian, from 0° to 180°. Lines of longitude are meridians: they run from pole to pole and meet at both ends.
There's no natural zero for longitude, so it had to be agreed. At the International Meridian Conference in Washington in 1884, most of the countries attending chose the meridian through the Royal Observatory at Greenwich, London. Heading south from Britain it crosses France, Spain, Algeria, Mali, Burkina Faso, Togo and Ghana before reaching Antarctica. Halfway round the world sits the 180° meridian, which the International Date Line roughly follows, zigzagging around island groups.
Because meridians meet at the poles, a degree of longitude shrinks as you move away from the equator: about 111 km at the equator, about half that at 60°N, and nothing at the pole. And because the Earth turns 15° every hour, longitude and time are tied together. That is what a sea clock like Harrison's was for: compare local noon with the time at home and the difference gives your longitude.
Writing coordinates
Latitude always comes first, then longitude. There are two common ways to write them:
- Degrees, minutes and seconds: 1° = 60′ and 1′ = 60″, with N, S, E or W after each number.
- Decimal degrees: north and east are positive, south and west are negative. Cape Town at roughly 33.9°S, 18.4°E becomes about −33.9, 18.4.
The point 0°, 0° lies in the Gulf of Guinea, off West Africa. Map makers nicknamed it Null Island: there's no land there, but lots of faulty data ends up plotted on it when coordinates go missing.
Reading a map
- Scale turns map distance into real distance. At 1:50,000, 1 cm on the map is 500 m on the ground.
- Contour lines join points of equal height. Close together means steep; far apart means gentle.
- Grid references give eastings (along) before northings (up). The old classroom rhyme is "along the corridor, then up the stairs".
- The key (or legend) explains the symbols, and the compass rose or north arrow shows direction.
Easy ones to mix up
- Latitude and longitude. Latitude lines look like the rungs of a ladder ("lat-itude, lad-der"); longitude lines are the long ones running pole to pole.
- Rome and New York. Rome, at about 41.9°N, is further north than New York, at about 40.7°N. Europe's mild climate fools people.
- Reno and Los Angeles. Reno, Nevada is further west than Los Angeles, because the California coast bends east as it runs south.
- Eastings and northings. Along first, then up.
Frequently asked questions
What is the difference between latitude and longitude?
Latitude measures north–south position from the equator (0° to 90°); longitude measures east–west position from the prime meridian (0° to 180°).
Which comes first, latitude or longitude?
Latitude. A coordinate such as 51.5°N, 0.1°W gives latitude, then longitude.
Why is Greenwich 0° longitude?
Because delegates at the International Meridian Conference in 1884 agreed on it. Most sea charts already used Greenwich, which helped it win.
How far is one degree of latitude?
About 111 km, or 69 miles. A degree of longitude is that long only at the equator and shrinks towards the poles.
Try it
Cover the answers and say which hemispheres each card's place sits in before you flip it. Then try this one on a globe: where in the world could you stand on the equator and the prime meridian at the same time, and what would you find under your feet?