Latitude, longitude, and time zones

How would you tell a friend exactly where to find a particular spot on Earth — say, the city of Quito in Ecuador, or a ship in the middle of the Pacific Ocean? Centuries ago, geographers invented an invisible grid of lines wrapped around the planet. Two numbers — a latitude and a longitude — pinpoint any place on Earth, no matter how remote.

Latitude — how far north or south

Lines of latitude are horizontal rings that circle the Earth parallel to one another. They are also called parallels. Latitude is measured in degrees from 0° at the equator to 90° north at the North Pole and 90° south at the South Pole. The equator itself is the longest latitude line; the parallels get shorter as you move toward either pole. Three famous parallels are the Tropic of Cancer (23.5° N), the Tropic of Capricorn (23.5° S), and the Arctic and Antarctic Circles (66.5° N and S).

Longitude — how far east or west

Lines of longitude — also called meridians — run vertically from the North Pole to the South Pole. Unlike parallels, meridians are not parallel: they pinch together at the poles and are widest at the equator. Longitude is measured east or west of the Prime Meridian, an arbitrary "zero" line that passes through Greenwich, near London, England. Longitude runs from 0° at Greenwich to 180° east AND 180° west, meeting on the opposite side of the planet in the middle of the Pacific.

Putting it together

Every place on Earth has a unique pair of coordinates written as latitude first, then longitude. Quito, Ecuador, sits very near 0° latitude on the equator, with a longitude of about 78° W. New York City is roughly 40° N, 74° W. Sydney is 33° S, 151° E. When you read coordinates on a map, latitude tells you the row and longitude tells you the column — together they pinpoint a single spot.

Time zones

Earth rotates 360° once every 24 hours, so it turns 15° per hour. To keep clocks roughly in sync with the sun, the world is divided into 24 time zones, each about 15° of longitude wide. Inside one zone, every clock reads the same time. As you cross from one zone into the next moving east, your clock jumps forward one hour; moving west, it falls back one hour. The reference time at the Prime Meridian is called Greenwich Mean Time (GMT) or, in scientific use, Coordinated Universal Time (UTC).

The International Date Line

On the opposite side of the planet from Greenwich, at roughly 180° longitude, runs the International Date Line. When you cross it traveling west, the calendar jumps forward by one day; crossing east, it jumps back one day. The line zigzags around several Pacific island groups so that whole countries are not split between two different dates. Without it, an airplane circling the globe would arrive at the wrong day no matter how carefully it counted the hours.

• Latitude — 0° at the equator, 90° at each pole; horizontal lines (parallels).
• Longitude — 0° at the Prime Meridian (Greenwich); 180° at the opposite side; vertical lines (meridians).
• Coordinates always written latitude first, then longitude.
• Earth rotates 15° per hour → 24 time zones spaced ~15° apart.
• Greenwich Mean Time (GMT) / UTC is the reference clock at 0° longitude.
• International Date Line at ~180° — cross west: skip a day; cross east: repeat a day.