Planes fly in an arc because the Earth is a sphere, and the shortest distance between two points on a sphere is a curve, not a straight line. What looks like an unnecessary detour on a flat map is actually the most direct route possible. This illusion comes from the way we flatten the round Earth onto rectangular maps, which warps distances and shapes, especially near the poles.
Great Circles: The Shortest Path on a Sphere
On a flat surface, the shortest path between two points is a straight line. On a sphere, the equivalent is called a great circle: the path you’d get if you sliced the Earth with a plane passing through its exact center. The resulting circle on the surface is the largest possible circle you can draw on a sphere, and any segment of it represents the shortest route between two points.
This is easy to see with a physical globe. Stretch a piece of string between New York and Tokyo, pulling it taut against the globe’s surface. The string will arc up over Alaska and the northern Pacific. That path looks bizarre on a flat map, but on the globe, you can see it’s genuinely the shortest connection. Flying a “straight” east-west line across the middle of the Pacific would actually cover hundreds of extra miles.
Why Flat Maps Make It Look Wrong
Most world maps use the Mercator projection, which was designed in the 1500s for sea navigation. It represents lines of constant compass bearing as straight lines, which made it easy for sailors to plot a course. The tradeoff is that it stretches land masses and distances dramatically as you move away from the equator, making Greenland look the size of Africa when it’s actually about 14 times smaller.
On a Mercator map, a straight line between two cities is called a rhumb line. It keeps a constant compass direction the entire way, which is convenient for navigation but is almost never the shortest path. A great circle route, by contrast, constantly changes its compass bearing as it curves across the map. That changing bearing is what creates the arc you see on a flat map projection. There’s actually a different type of map, the gnomonic projection, that shows all great circle routes as straight lines, but it distorts shapes so badly it’s rarely used outside of flight planning.
How Much Distance This Actually Saves
The savings are real and significant. A study of Chinese domestic flights between 2019 and 2023 found that switching to great circle routes during the cruising phase could reduce flight distances by up to 16.8% on certain routes, with fuel savings of roughly 9.6% across all analyzed flights. That translated to 9.3 million metric tons of fuel and 29.4 million metric tons of CO₂ emissions over the study period.
The effect is strongest on long east-west routes at higher latitudes, where the Mercator distortion is greatest. A flight from London to Los Angeles, for example, curves far north over Greenland and northern Canada rather than crossing the Atlantic at a lower latitude. For short flights near the equator, the difference between a great circle and a straight map line shrinks to almost nothing, which is why you mostly notice the arc on long-haul international routes.
Wind Patterns Reshape the Route
In practice, planes don’t follow a perfect great circle. Pilots and dispatchers adjust the route to account for high-altitude winds, particularly the jet stream, a band of fast-moving air that flows west to east at cruising altitude. Over the North Atlantic, the jet stream can blow at over 200 mph.
Eastbound flights from North America to Europe are often routed to ride the jet stream’s tailwind, shaving significant time off the trip. Westbound flights do the opposite, diverting south or north to avoid flying into that same headwind. This is why a flight from New York to London is typically an hour shorter than the return trip. The great circle is the starting point for route planning, but wind optimization can push the actual path noticeably north or south of it.
Emergency Airport Rules Add More Curves
Safety regulations also pull flights away from the theoretically perfect arc. Under rules known as ETOPS (Extended Operations), a twin-engine plane must stay within 60 minutes of flying time from a suitable emergency airport at all times, unless the airline has special approval to extend that limit. Airlines can earn certification for 120, 180, or even 240 minutes of diversion time, but each level requires additional maintenance standards and crew training.
This matters most over oceans. A great circle route from the U.S. to Asia might pass over vast stretches of the Pacific with no airports below. If an airline’s ETOPS rating doesn’t cover that distance, the flight will curve toward landmasses with suitable diversion airports, adding miles but keeping passengers within reach of an emergency landing site. Planes with three or four engines face a less restrictive threshold of 180 minutes, which is one reason older wide-body jets with four engines were once preferred for transoceanic routes.
Airspace Restrictions and Geopolitics
National airspace adds another layer of deviation. Countries control the skies above their territory, and airlines need permission to fly through. Political conflicts can close off entire corridors. When Russian airspace is unavailable, for instance, flights between Europe and East Asia must reroute far to the south, adding hours to trips that would otherwise arc over the Arctic. Military zones, conflict areas, and countries that charge high overflight fees all nudge routes away from the ideal great circle path.
The arc you see on your seatback screen is the result of all these forces combined: the geometry of a spherical planet, the distortion of flat maps, wind patterns at 35,000 feet, emergency landing requirements, and the political landscape below. The great circle sets the ideal, and everything else adjusts it. But the fundamental reason the path looks curved is simple. The Earth is round, and your map is flat.