Pull up a flight tracker for a trip from New York to Tokyo and something looks wrong. Instead of heading west in a tidy straight line, the plane arcs north, skimming past Alaska and the edge of the Arctic before dropping down into Japan. The pilot is not lost. The plane is following a great circle, the genuinely shortest path across a round planet, even though it looks like a detour on a flat map.
Straight Lines Lie on Flat Maps
A great circle is any circle drawn on a sphere whose centre is the centre of the sphere itself. The equator is a great circle, and so is every line of longitude. Crucially, the shortest distance between any two points on a globe always lies along the great circle that connects them. On a globe this is obvious: stretch a string between two cities and it naturally hugs the surface along the shortest route.
Why the Shortest Route Curves
The trouble starts when you flatten the globe onto a map. The familiar Mercator projection stretches the high latitudes enormously, which is why Greenland looks as big as Africa. That same stretching takes a great circle, which in reality bows toward the nearest pole, and draws it as a curved arc. A path from London to Los Angeles really does pass near Greenland; a path from New York to Tokyo really does run close to the Arctic. The curve on the screen is the straight, efficient route in disguise.
The Rhumb Line Trade-Off
There is another kind of route, called a rhumb line, that crosses every meridian at the same angle. On a Mercator map a rhumb line is a perfectly straight line, which is exactly why the projection was invented: a sailor could draw a straight pencil line and hold a single constant compass bearing all the way. The catch is that a rhumb line is longer than a great circle. For centuries, navigators happily traded a few extra miles for the simplicity of not constantly re-checking their heading.
- New York to Tokyo: arcs north past Alaska and the Aleutians
- London to Los Angeles: curves up over Greenland and northern Canada
- Sydney to Santiago: dips far south, skirting Antarctic waters
- Each looks like a detour on a flat map, yet each is the shortest real path
See It for Yourself
The cleanest demonstration is a globe and a piece of string. Pin one end on your home city, pull the string taut to a distant city, and watch the route bend toward the pole. Online great-circle mapping tools do the same thing digitally, drawing the true shortest path and then showing how badly a flat map distorts it.