Why a Flight from New York to Tokyo Goes Through Alaska

Pull up a flight tracker for a New York-Tokyo route. The plane heads northwest over Canada, brushes near the Aleutian Islands, and turns south toward Japan. On a flat map projection it looks like a 1,500-mile detour. On a globe it is the most direct path possible.

The shortest distance between two points on a sphere lies along a great circle — a circle whose plane passes through the center of the sphere. Earth's equator is a great circle. Every line of longitude is half of one. Lines of latitude, except for the equator, are not. A great circle between two cities at the same latitude usually arcs north (in the Northern Hemisphere) toward the pole rather than running flat along the parallel.

The distortion is built into flat maps. The Mercator projection, designed in 1569 by Gerardus Mercator for marine navigation, preserves angles and bearings — useful for plotting a constant-heading course — at the cost of badly distorting distances and area. Greenland looks larger than Africa; it is actually about one-fourteenth the size. Routes that look curved on Mercator are usually the straight ones in three dimensions.

Long-distance ships and planes have used great-circle routing since the 19th century, when it became practical to compute the bearing changes needed along the way. Modern flight management systems do the math continuously, adjusting heading every few miles. The route looks like a detour because the map is the lie. The world is round, and the planes know it.