Definition
Latitude and longitude are imaginary lines drawn across the Earth's surface so any spot can be described exactly. Latitude, the horizontal lines, tells you how far north or south you are from the equator. Longitude, the vertical lines, tells you how far east or west you are from the prime meridian.
Latitude: the horizontal lines that shape sunlight and climate
Think of counting rows on a game board. The imaginary lines that run around the Earth horizontally are called parallels, or lines of latitude. The most important one is the equator, the Earth's waistline, at 0 degrees latitude. Going north, the numbers grow up to 90 degrees north (the North Pole). Going south, they grow up to 90 degrees south (the South Pole).
Latitude doesn't just give you a location. It also has a big effect on a place's climate. Near the equator, the sun shines almost straight down, so the climate is hot and tropical. Toward the poles, sunlight hits at a slant, temperatures plunge, and you get a frozen polar climate.
Seoul, South Korea, sits at about 37.5 degrees north. That latitude is a big reason it has a temperate climate with four distinct seasons. Its hot, humid summers and fairly cold winters also come from seasonal winds and the influence of the Asian continent.
Longitude: the vertical lines behind clocks and time zones
The vertical lines that wrap around the Earth like the segments of an orange are called meridians, or lines of longitude. Longitude starts at the prime meridian (0 degrees), which runs through the old Royal Observatory in Greenwich, London. From this line, you measure up to 180 degrees east in one direction and 180 degrees west in the other.
Longitude is the key to how the world sets its clocks. The Earth spins once (360 degrees) every 24 hours, so there is a one-hour time difference for every 15 degrees of longitude. Places east of the prime meridian see the sun first, so their clocks run ahead. Places to the west run behind.
South Korea actually lies around 127 degrees east. For convenience, though, it sets its standard time by 135 degrees east. That puts Korean clocks 9 hours ahead of Coordinated Universal Time (UTC), which is based on 0 degrees longitude. While the UK is on daylight saving time in summer, the gap with the UK shrinks to 8 hours.
A closer look: how GPS works and what degrees, minutes, and seconds mean
Once you know the single point where a line of latitude crosses a line of longitude, any place on Earth has its own one-of-a-kind address. That's how the map app or navigation system on your phone can find you down to the building entrance, thanks to latitude and longitude coordinates calculated from satellite signals. The satellites don't actually send coordinates. They send precise timing and orbit data, and your phone works out its distance to several satellites to calculate its own position.
The Earth is huge, so even one degree of latitude covers about 111 kilometers (about 69 miles). To pin down an exact address, each degree is split into 60 minutes, and each minute into 60 seconds. This is the degrees-minutes-seconds system, and it's used alongside decimal degrees.
On a flat map, the lines look like a neat grid. On a real globe, they look different. Parallels are circles that get smaller toward the poles, but meridians are arcs that all meet at a single point at the North and South Poles.
Common misconceptions
Lines of longitude, like lines of latitude, all have different lengths and get shorter toward the poles.
Parallels (horizontal lines) shrink into smaller circles toward the poles. Meridians (vertical lines) all pass through both the North and South Poles, so they all have the same length, about 20,000 kilometers (about 12,400 miles) as a half circle.
One degree of latitude and one degree of longitude always cover the same real distance, anywhere on Earth.
One degree of latitude is almost constant at about 111 km (69 miles). One degree of longitude is about 111 km at the equator, but it gets narrower toward the poles and shrinks to 0 meters at the North and South Poles.
Where you meet it
Airplane pilots and ship navigators find their way across open ocean using coordinates like "35 degrees north, 129 degrees east."
When you tap "Share my location" with a friend on your phone, live latitude and longitude numbers are sent in the background.
In one sentence
Latitude, the horizontal lines, divides the world by climate. Longitude, the vertical lines, divides it by time zone. Where the two cross, they pinpoint every spot on Earth.