Latitude and longitude are two sets of imaginary lines that form a grid over the whole Earth. Latitude lines run sideways and measure how far north or south of the equator a place is, from 0 degrees at the equator to 90 degrees at each pole. Longitude lines run from pole to pole and measure how far east or west of the Prime Meridian a place is, up to 180 degrees. Put the two numbers together and every spot on the planet has its own address.
Earth's Invisible Grid in One Minute
Think of the game Battleship. The board has letters along one side and numbers along the other, and you call out "B7" to name one square. Latitude and longitude do the same job for the whole planet, except the board is a sphere and the squares are measured in degrees instead of letters. A degree is simply one 360th of a full circle, the same unit you use for angles in maths.
Latitude tells you how far up or down the globe you are. Longitude tells you how far round. Written together as a pair of numbers they are called coordinates, and latitude always comes first. Give someone the coordinates 48.86 degrees north, 2.35 degrees east and they can find the Eiffel Tower without knowing the name of a single street.
Latitude: The Ladder Lines from the Equator to the Poles
Lines of latitude run east to west, parallel to each other like the rungs of a ladder, which is why they are also called parallels. A handy memory trick: "lat" sounds like "flat", and the lines lie flat across the map.
The starting line is the equator, at 0 degrees, which runs around the fattest part of the Earth about 40,075 kilometres long. From there the numbers climb to 90 degrees north at the North Pole and 90 degrees south at the South Pole, so the letter N or S after the number tells you which half of the world you are in. Four other parallels are so important that they are drawn on most globes:
- Tropic of Cancer, about 23.4 degrees north: the furthest north the sun is ever directly overhead, on the June solstice.
- Tropic of Capricorn, about 23.4 degrees south: the same for the December solstice. The band between the two tropics is the tropics, the hottest part of the world.
- Arctic Circle, about 66.6 degrees north: north of here the sun does not set on at least one day in summer.
- Antarctic Circle, about 66.6 degrees south: the same rule around Antarctica.
Because the parallels are evenly spaced, one degree of latitude is always about 111 kilometres on the ground. Each degree is split into 60 minutes, and one minute of latitude is exactly the distance sailors call a nautical mile, 1,852 metres. That is why ships and planes still measure speed in knots, or nautical miles per hour. Our post on the equator explains why the starting line sits where it does.
Longitude: The Lines That Meet at the Poles
Lines of longitude run north to south from pole to pole and are called meridians. Unlike parallels they are not parallel at all: they are widest apart at the equator and all meet at the two poles, like the segments of an orange. "Long" lines are the long ones, running top to bottom.
Nature gave us an obvious zero line for latitude, the equator, but there is no natural zero for longitude. Every country used to pick its own. In October 1884 delegates from 25 nations met at the International Meridian Conference in Washington, D.C., and voted to make the meridian through the Royal Observatory at Greenwich, in London, the Prime Meridian at 0 degrees. Britain already printed most of the world's sea charts, so most ships were using it anyway. France abstained and kept using Paris on its own maps for another few decades.
From Greenwich the numbers run to 180 degrees east and 180 degrees west, which meet on the opposite side of the world in the middle of the Pacific Ocean. That line, with a few zigzags around islands, is the International Date Line. Cross it heading west and the calendar jumps forward a day. Longitude is also tied to time: Earth turns 360 degrees in 24 hours, so it turns 15 degrees every hour, which is why time zones are roughly 15 degrees wide. You can read about the zero line in our guide to the Prime Meridian and about the jump in the calendar in the International Date Line explained.
One degree of longitude is about 111 kilometres at the equator, but it shrinks as you go toward the poles because the meridians squeeze together. At the latitude of London it is about 69 kilometres, and at the North Pole it is zero, since all the lines meet in one spot. Stand exactly on the pole and you are on every line of longitude at once.
Reading Coordinates Like a Treasure Map
Coordinates can be written two ways. The old way uses degrees, minutes and seconds, like 40 degrees 42 minutes 46 seconds north. The modern way, used by phones and GPS, uses decimals: 40.7128 degrees north. Both mean the same thing. Here are some real addresses to try on a globe:
- New York City: 40.7 N, 74.0 W. North of the equator, west of Greenwich.
- Sydney: 33.9 S, 151.2 E. South and far to the east.
- Cairo: 30.0 N, 31.2 E. Almost the same latitude as New Orleans, which is why both are warm.
- Quito, Ecuador: 0.2 S, 78.5 W. So close to the equator that a monument outside the city marks the line, and the country is named after it.
- The North Pole: 90 N, with no longitude needed.
- Null Island: 0 N, 0 E. Where the equator crosses the Prime Meridian, in the Gulf of Guinea off West Africa. There is no island, just a weather buoy, but map software that loses a location often drops it here by mistake, so geographers joke that it is the most visited place in the world that does not exist.
Two places with the same latitude get the same length of daylight on any given day. Two places with the same longitude have noon at the same moment. That is why latitude is the number that matters for climate, and longitude is the one that matters for clocks.
Why Longitude Was So Hard to Find: John Harrison's Clocks
Sailors could work out their latitude for centuries by measuring how high the sun or the North Star stood above the horizon. Longitude was a different story. On 22 October 1707 four British warships, lost in fog and unsure how far east they had sailed, ran onto rocks off the Isles of Scilly and about 1,400 sailors drowned. In 1714 the British Parliament passed the Longitude Act, offering a prize of up to 20,000 pounds, a vast fortune at the time, to anyone who could solve the problem.
The answer was a clock. Because Earth turns 15 degrees an hour, a ship that knew the exact time back in Greenwich could compare it with local noon, measured from the sun, and convert the difference into degrees of longitude. The trouble was that pendulum clocks were useless on a rolling, damp, hot-and-cold ship. A self-taught carpenter named John Harrison spent most of his life building a clock that would keep time at sea. His fourth attempt, the H4, was a large pocket watch that lost only about five seconds on an 81-day voyage to Jamaica in 1761 and 1762. Harrison was 80 before he finally received most of his prize money. Modern GPS, explained in our post on how GPS works, still uses his idea: it finds your longitude by comparing extremely accurate clocks, except now the clocks are on satellites.
Latitude and Longitude Versus a Street Address
If a postal address can get a parcel to your door, why bother with coordinates? Because the two systems are good at different things.
- Coverage. A street address only exists where someone has built a street. Coordinates work in the middle of the Sahara, on the Antarctic ice, at sea and on a mountaintop. Rescue helicopters are sent to coordinates, not to addresses.
- Precision. An address names a building. Coordinates to four decimal places pin down a spot about 11 metres across, and six decimal places get you to about 11 centimetres.
- Language. Addresses are written in dozens of alphabets and formats. A pair of numbers means the same thing to a pilot from Brazil and a sailor from Japan.
- Human friendliness. Here the address wins. Nobody remembers that their school is at 51.5014 N, 0.1419 W, and telling a taxi driver your coordinates would earn you a strange look.
So the two work as a team. Your phone's map app stores every place as coordinates behind the scenes and translates them into street names for you.
Three Coordinate Games to Try
- Find your house. Open any online map, press and hold on your home, and read off the coordinates. Write them down, then find the coordinates of your school and work out whether school is north, south, east or west of you from the numbers alone.
- Same-latitude twins. Pick a city and run your finger along its parallel on a globe. What other cities share it? Madrid, New York and Beijing all sit near 40 degrees north. Which has the coldest winters, and why might that be, if latitude is the same?
- Coordinate treasure hunt. One player writes down the coordinates of a famous landmark. The others have 60 seconds with an atlas or a globe to name it. Start easy, with 27.2 S, 109.4 W, which is Easter Island, and work up to harder ones.
Once the grid makes sense, the next step is seeing how it divides the world into halves, which our hemispheres for kids guide takes on.