Why the shortest flight looks curved on a map

Look at a flight from London to New York on a flat world map and the route bows north, over Ireland and Newfoundland, instead of heading straight across the Atlantic. The plane isn't taking a detour. On a globe that bowed line is the shortest way, and the “straight” line on the map is the long way round.

Great circles

The shortest path between two places on a sphere follows a great circle: a circle around the globe whose centre is the centre of the Earth, like the equator or any line of longitude. Stretch a piece of string tight between two cities on a desk globe and it lies along one. Flatten the globe into a map and those arcs bend towards the nearest pole, which is why long flights seem to curve north in the northern hemisphere.

The effect grows with distance and with latitude. The shortest route from New York to Beijing runs north over Canada, close to the Arctic Ocean, and down through Siberia, which is why airlines fly polar routes between North America and Asia.

Some real distances

Two of these surprise most people. Lisbon to Moscow is about as far as New York to Los Angeles: Europe, coast to its eastern edge, is roughly as wide as the United States. And London to New York, which feels far, is less than a third of the way to Sydney.

The farthest you can go

The Earth is about 40,075 km around at the equator, so no two places are more than about 20,000 km apart: the distance from pole to pole is 20,004 km. The point exactly opposite you on the globe is your antipode. Madrid's is in New Zealand's North Island, near the small town of Weber, which is why Madrid to Wellington, at 19,854 km, is close to the longest trip you can make between two capitals.

Why flat maps mislead

The most familiar world map, the Mercator projection, was designed for sailors. Any line of constant compass bearing is straight on it, which made it ideal for steering by compass. The price is distortion away from the equator: Greenland looks as big as Africa, though Africa is about 14 times larger. Straight lines on a Mercator map are not the shortest routes either, which is why measuring distances on one leads you astray.

How to guess

Distance Guesser scores on how many times too far or too short you are, so anchors matter more than precision. Keep a few in mind: about 4,000 km across the United States or across Europe, about 5,600 km across the North Atlantic, about 10,000 km from Europe to Japan, and about 20,000 km to the far side of the world.