Definition
A great-circle route is the shortest flight or sailing path between two points on the round Earth. It follows a great circle: the biggest circle you get on the surface when you slice the Earth straight through its center with a giant knife.
How to Avoid Being Fooled by a Flat Map
Spread out a world map and lay a ruler from Seoul to New York. The straight line cuts right across the middle of the Pacific Ocean. But turn on the flight tracker on a real plane, and it doesn't fly across the middle of the Pacific. It arcs high up through the northern sky.
At first glance, you might wonder why the plane doesn't just go straight and instead takes a long detour. But that's an illusion, because we're so used to flat, two-dimensional paper maps. The Earth isn't a flat sheet of paper. It's a round, three-dimensional ball.
Place a string on a globe and pull it tight between Seoul and New York, and you'll notice something surprising. The northern arc, passing the eastern tip of Siberia, near the Arctic Ocean, and northern Canada, is much shorter than the Pacific crossing that looked like a straight line on the flat map. Flights to New York today head toward Alaska partly because, since 2022, they avoid Russian airspace and stay as close to this arc as they can.
A paper map is made by stretching a round Earth flat, so the closer you get to the North Pole, the more land and sea look blown up far beyond their real size. That's why the route can look like a long way around to your eyes. On the actual round surface of the Earth, though, this northern route is the fastest shortcut.
The Biggest Circle You Can Cut Around the Earth
Cut a whole watermelon in half, right through the exact center, and the cut edge forms the largest round circle possible. In the same way, when you slice the Earth with a flat plane that passes through its center, the biggest circle that appears on the surface is called a great circle. If you cut off-center instead, you get a much smaller circle.
On a globe, the equator and every line of longitude running north to south are classic great circles, because they all go through the Earth's center. The other lines of latitude, on the other hand, are just smaller circles that miss the center and sit above or below it.
To fly between two places using the least fuel and time, you imagine one giant circle that passes through both cities and the center of the Earth, then travel along an arc of that circle. This is the geometry behind great-circle routes, the method long-haul international flights and large ships use.
If you leave this path and follow a line that only looks straight on a map, you actually travel a much longer distance. That wastes thousands of liters of jet fuel and precious flying time.
A Little More Precisely
When a ship or plane follows a great-circle route, the pilot has to keep turning the compass heading a little at a time. A great-circle route crosses the lines of longitude at a slant, so the angle between the route and the compass needle keeps changing in real time throughout the trip.
A path that keeps one fixed compass angle the whole way is called a rhumb line. In the age of sailing ships, a rhumb line was easier to steer because the heading never changed. But it ends up much longer, so modern aircraft use computers to keep adjusting the heading and fly a great-circle route as their baseline.
Airlines also don't always insist on the mathematically perfect great-circle line. They use it as the basic frame, then make small adjustments all the time. They might ride a strong jet stream (fast winds high in the sky) that pushes the plane from behind, or steer around dangerous storms.
In the end, the great-circle route is the most economical baseline that geometry has ever given us. By understanding the round curve of the Earth, we save a huge amount of time and energy in the sky every day.
Common misconceptions
Planes curve north because they are taking a long way around to avoid bad weather over the Pacific.
Because the Earth is a sphere, the route that bulges up to the north is itself the shortest path. That said, since 2022 the Alaska-ward path of New York flights has also been shaped partly by avoiding Russian airspace.
Where you meet it
When you fly from Incheon Airport to San Francisco, the plane doesn't cross the middle of the Pacific. It passes near northern Japan and Alaska.
The shortest great-circle route from Seoul to London or Paris isn't a straight line west. It goes northwest over the skies of Siberia. Since 2022, flights have detoured toward China and Central Asia to avoid Russian airspace, and flight times have sometimes gotten longer.
In one sentence
It's the shortest, fastest path connecting two points on the surface of the round Earth.