What Is a Great Circle Route? Why Flight Paths Look Curved
Pull up the moving map on a long flight and you will notice something odd. The little plane is not flying in a straight line to its destination. On the India to North America routes I take, it curves up towards the North Pole, drifts over Russia or Greenland, then comes down the far side. It looks like a detour. It is actually the shortest way there.
So what is a great circle?
A great circle is any circle you get by slicing a sphere straight through its centre. The equator is one. The shortest distance between any two points on a globe always lies along the great circle that joins them. Stretch a piece of string tight between two cities on a real globe and it traces a great circle. That taut string is the path a long-haul flight wants to follow.
Why the shortest path looks bent
The confusion comes from the map, not the route. A world map is a flat sheet, but the Earth is a ball, and you cannot flatten a ball without stretching it somewhere. Most flat maps stretch the areas near the poles enormously, which is why Greenland can look as big as Africa when it is really a fraction of the size. On that stretched sheet, the genuinely shortest path gets pulled into a curve. Put the same two cities on a globe and the curve straightens out into the obvious shortest line.
You have probably already seen it
This is why a flight from Delhi to New York or Toronto heads north over Central Asia and the Arctic instead of running flat across the Middle East and the Atlantic. The polar route really is shorter. The seat-back map shows the curve, the airline planned the curve, and the fuel was loaded for the curve.
Which routes bend the most
How much a route curves depends on where it goes. Long flights that run mostly east to west at high latitudes bend the most, so Delhi to the United States or to northern Europe arcs clearly over the top of the world. Routes that run mostly north to south barely bend at all. A Delhi to Dubai or Delhi to Singapore flight looks almost straight on any map, because there is little distortion to fight. Short flights of any direction look straight too, simply because there is not enough distance for the curve to show.
The distance you see is the great-circle distance
When the calculator on this site gives you a distance, that is the great-circle distance, measured the way the aircraft actually flies, not a ruler line across a flat map. Run Delhi to New York and you get about 11,755 km. Try a few high-latitude routes and watch how far north the arc climbs. Open the calculator and the map under each result draws the very curve you just read about.
So the next time the moving map shows your plane looping up towards the pole, you can sit back. It is not lost. It is taking the only shortest path a round world allows.