How latitude and longitude work
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Greenwich, in London, is at about 51.5° N, 0°. Sydney is at about 33.9° S, 151.2° E. New York is at about 40.7° N, 74.0° W. Two numbers, each with a letter, are enough to pin down any place on Earth, once you know what they measure.
Latitude: how far north or south
Latitude is the angle north or south of the Equator, measured from the centre of the Earth. The Equator is 0°, the North Pole 90° N and the South Pole 90° S. Lines of latitude are parallel to one another, which is why they are also called parallels.
Because the parallels are evenly spaced, one degree of latitude is about 111 km wherever you are.
Five parallels have names. They come from the tilt of the Earth's axis, about 23.4°.
| Line | Latitude (approx.) | What it marks |
|---|---|---|
| Arctic Circle | 66.6° N | North of here, at least one day a year without a sunset |
| Tropic of Cancer | 23.4° N | Furthest north the Sun is ever directly overhead |
| Equator | 0° | Halfway between the poles |
| Tropic of Capricorn | 23.4° S | Furthest south the Sun is ever directly overhead |
| Antarctic Circle | 66.6° S | South of here, at least one day a year without a sunset |
The connection with the tilt is explained in seasons, eclipses and tides.
Longitude: how far east or west
Lines of longitude, called meridians, run from pole to pole. They are not parallel: they are widest apart at the Equator and meet at the poles. All of them are the same length.
Nature gives no obvious starting line for longitude, so one had to be agreed. At an international conference in Washington, D.C., in 1884, most of the countries attending chose the meridian through the Royal Observatory, Greenwich, as 0°: the prime meridian. By then most of the world's sea charts already used Greenwich, so the choice meant fewer charts had to change.
Longitude runs from 0° to 180° east or west. Latitude stops at 90° because the pole is a quarter turn from the Equator; longitude reaches 180° because it goes halfway round the world in each direction. The 180° meridian lies in the Pacific, and the International Date Line roughly follows it.
Reading and writing coordinates
By convention latitude comes first. "40.7° N, 74.0° W" means 40.7 degrees north of the Equator and 74.0 degrees west of Greenwich.
Degrees divide like hours:
- 1 degree = 60 minutes (′)
- 1 minute = 60 seconds (″)
Worked example: decimals to minutes
Convert 51.5° to degrees and minutes.
- Keep the whole number: 51°.
- Multiply the decimal part by 60: 0.5 × 60 = 30.
- 51.5° = 51° 30′.
Watch out for 51.30°. It is not 51° 30′: 0.30 × 60 = 18, so 51.30° = 51° 18′.
Phone maps usually show decimals and write south and west as negative numbers. So −33.9, 151.2 means 33.9° S, 151.2° E. Some computer formats put longitude first, so always check which number is which.
Distances on the grid
Two towns lie on the same meridian, one at 50° N and the other at 54° N.
- Difference in latitude: 54 − 50 = 4°.
- Distance: 4 × 111 ≈ 444 km.
East–west distances are harder, because meridians squeeze together towards the poles. A degree of longitude is about 111 km at the Equator, about 56 km at 60° N, and nothing at all at the pole. The North Pole is simply 90° N: every meridian passes through it, so it has no single longitude.
Finding latitude from the sky
In the Northern Hemisphere, the Pole Star, Polaris, sits almost directly above the North Pole. Its angle above the horizon is roughly equal to your latitude: overhead (90°) at the North Pole, on the horizon (0°) at the Equator, and about 51° up from London. Our stargazing guide shows how to find it.
South of the Equator there is no bright pole star, so navigators measured the height of the Sun at midday and used printed tables to turn that into latitude.
Finding longitude: a problem of time
The Earth turns 360° in 24 hours, which is 15° every hour. So if you know the time at Greenwich and your own local time, the difference tells you your longitude.
Worked example
A navigator sees the Sun at its highest point, local noon, when a clock keeping Greenwich time reads 16:00.
- Local noon is 4 hours later than noon at Greenwich.
- 4 × 15° = 60°.
- The Sun reaches you later, so you are west of Greenwich: 60° W.
If local noon came at 09:00 Greenwich time instead, you would be 3 hours ahead, so 3 × 15° = 45° E.
The hard part was the clock. A pendulum clock is useless on a rolling ship, and changes of temperature upset most mechanisms. In the 1700s the English clockmaker John Harrison built a series of marine timekeepers; his fourth, H4, kept time well enough on a voyage to the Caribbean to show that the method could work. Navigators also used a second method, measuring the angle between the Moon and selected stars, but it needed long calculations.
Before then, sailors could estimate latitude fairly well but often had only a rough idea of their longitude. That was true of the crews on the Magellan–Elcano expedition.
Today a phone works out its position from the timing of signals sent by several satellites, and reports it on the same grid of latitude and longitude that sailors used.
Easy ways to slip up
- Mixing up the words. Lines of latitude lie flat like the rungs of a ladder; lines of longitude are long, from pole to pole.
- Leaving out N, S, E or W. "40°, 74°" could be four different places. 40° S, 74° W is in the Pacific just off the coast of Chile.
- Calling the Equator a line of longitude. It is the 0° line of latitude.
Activity: coordinate detective
Use an atlas or, with an adult's permission, a map app.
- Find the latitude and longitude of your town to the nearest whole degree.
- Find a place on another continent at the same latitude. What is its weather like in January?
- Convert to degrees and minutes: 48.25°, 12.75°, 35.1°.
- Local noon happens at 14:00 Greenwich time. What is your longitude?
(Answers: 3. 48° 15′, 12° 45′, 35° 6′. 4. 2 hours later than Greenwich, so 30° W.)
Sources and further reading
- Latitude and longitude – Britannica – how the grid is defined and measured.
- Longitude found: the story of Harrison's timekeepers – Royal Museums Greenwich – the longitude problem and how Harrison's clocks helped to solve it.
