AirportsGreat circle routes
Look at the moving map on a long flight and the aircraft seems to take a strange, curved path. From Delhi to the United States it heads north towards Russia and the Arctic, not west across Europe and the Atlantic. The pilot is not taking a detour. That curve is the shortest way.
The Earth is a ball, the map is flat
Stretch a string between two points on a globe and pull it tight: the string marks the shortest path. That path is part of a great circle, a circle whose centre is the centre of the Earth, like the equator or any line of longitude. A flat map cannot show the surface of a ball without stretching it, and most wall maps stretch the far north and south the most. A straight line drawn on such a map is not the shortest path on the real Earth, and the true shortest path looks bent.
Why the path bends towards the pole
In the northern half of the world, the great circle between two places at similar latitudes always bulges to the north, because the Earth is narrower there: the circles of latitude get shorter towards the pole, so cutting across the top saves distance. The farther apart the two places are, the farther north the path climbs. In the southern half of the world the bulge is to the south.
Worked out for some long routes
This table is calculated from the airport coordinates in our data. "Sets off towards" is the compass direction at the start of the shortest path. "Farthest north" is the highest latitude the path reaches.
| Route | Distance | Sets off towards | Farthest north on the way | Latitude of start and end |
|---|---|---|---|---|
| Delhi to New York JFK | 11,755 km | north-west (337°) | 70° N | 29° N and 41° N |
| Delhi to San Francisco | 12,382 km | north (16°) | 76° N | 29° N and 38° N |
| Mumbai to London Heathrow | 7,213 km | north-west (319°) | 51° N | 19° N and 51° N |
| Dubai to Los Angeles | 13,400 km | north (354°) | 85° N | 25° N and 34° N |
| Singapore to New York JFK | 15,341 km | north (357°) | 87° N | 1° N and 41° N |
| Delhi to Sydney | 10,435 km | south-east (127°) | 29° N | 29° N and 34° S |
| Tokyo Haneda to London Heathrow | 9,592 km | north-west (336°) | 71° N | 36° N and 51° N |
Notice that a flight from Delhi to the west coast of America starts out heading almost due north, although its destination is on the far side of the world to the east or west, depending on which way you count.
Why real flights do not follow it exactly
The great circle is the starting point for planning, not the final route. Airlines bend it to ride strong tail winds or avoid head winds, which can save more time than the extra distance costs. They must also stay within reach of airports where they could land in an emergency, keep out of airspace that is closed or expensive to cross, and follow the fixed tracks that air traffic control lays down over oceans. The path flown is usually a few percent longer than the great circle.
The compass heading keeps changing
Along a great circle the direction of travel changes all the time, even though the aircraft is flying "straight". A flight that leaves heading north may arrive heading south. Before computers, navigators broke the route into short legs, each flown on one steady heading.
Try it
The distance calculator gives the great-circle distance between any two airports on this site. A globe and a piece of string will show you the path.