Why Flight Paths Curve on Maps: Great Circles, Projections and Wind
Key takeaways
- Earth is a sphere, so the shortest route between two cities is a great circle arc, not a straight line on a flat map.
- Flat maps, especially the Mercator projection, stretch the poles and turn those arcs into dramatic curves.
- Jet streams, weather, and safety planning bend real flight paths further away from the geometric ideal.
On the seat-back screen, a flight from Los Angeles to London swoops north over Canada and Greenland, looking like an enormous detour. It is not a detour at all. That arc is the shortest path on a round planet, drawn on a flat rectangle that cannot represent it honestly.
This article explains why flight paths look curved on maps, what great circle routes are, how map projections create the illusion of a detour, and why wind and safety bend real routes even further.
What does "curved" really mean on a map?
Ask a classroom of mathematics students for the quickest way from A to B and they will answer: a straight line. That answer is true on a flat sheet of paper. Earth is not flat (despite what some apparently believe, as Simple Flying dryly notes), so a straight line drawn on a two-dimensional map is not the shortest way across a three-dimensional planet.
The curved route you see on the screen is curved because of the surface you are viewing it on. The map is a projection: a compromise that flattens the globe, and every compromise distorts something.
Most seat-back maps and school maps use the Mercator projection, which stretches distances near the poles in order to preserve angles and direction for navigation. On that projection, the shortest physical route appears as a sweeping curve that arcs toward the poles.
Great circle routes: the shortest path on a sphere
The shortest distance between two points on a sphere is a segment of a great circle, also called an orthodrome or geodesic. A great circle is any circle on the sphere whose center is the planet's center: the equator is one, and any line of longitude paired with its opposite meridian is another.
Stretch a string tightly across a globe between two cities and the line it forms is the great circle route. On that globe it looks "straight." Flatten it onto a rectangular map and the same path appears bent, sometimes dramatically.
Because Earth bulges at the middle, curving toward the poles shortens the distance even more than a first glance suggests. As GIS Geography has highlighted, the distance around Earth from pole to pole is shorter than around the equator. The curvature of Earth and its extra equatorial width mean that curving toward the poles covers less ground than following what looks like a straight line.
That is why a Los Angeles to London flight appears to fly up over Canada and Greenland. It is not wandering. It is hugging the great circle.
Airlines follow these routes to save thousands of pounds of fuel and significant amounts of time on transcontinental flights, since fuel is one of the largest operating costs in aviation.
Map projections: why the Mercator map bends straight lines
The Mercator projection was designed for sailors who needed to hold a constant compass heading. It preserves angles and direction beautifully, but it pays for that by inflating the far north and south. Greenland looks as large as Africa on a Mercator map; in reality it is about one-fourteenth the size.
That inflation is exactly what makes great circle routes look so exaggerated. Near the poles, where great circle routes naturally swing, the map stretches everything sideways. A route that is a gentle arc on the globe becomes a dramatic bow on the screen.
If instead you followed a constant compass heading, called a rhumb line or loxodromic path, the route would draw as a straight line on a Mercator map but would actually be longer and less efficient, burning more fuel for no reason. Pilots choose the curved-looking great circle because it is the short path in the real world, not on the rectangle.
Wind, weather, and safety: why routes bend beyond geometry
The great circle is the starting point, not the final answer. Modern flight planning software takes the geometric ideal and bends it further to account for the atmosphere. Geometry gives the shortest physical distance, but it ignores the fluid dynamics of the air the plane actually flies through.
The biggest modifier is the jet stream. These high-altitude air currents flow near the top of the troposphere, where shifting temperatures and pressures create wind tunnels that can top 200 mph, with regular speeds of 80 to 140 mph quite common.
Earth has four main jet streams, two per hemisphere, and because of the planet's rotation they mostly flow west to east. That is the same physics behind why airports plan around wind when choosing runways.
Flying with the jet stream shaves time off a flight, while flying into it slows the plane down. So westbound routes often detour further from the great circle to dodge a headwind, while eastbound flights ride the stream and may arrive early. The curve on your screen therefore reflects both the shape of Earth and the mood of the upper atmosphere that day.
Safety plays a role too. When planning a route, many pilots prefer to maximize the number of airports along the flight path.
If an engine fails over the middle of the ocean or a passenger has a medical emergency, knowing where you can land matters enormously. ETOPS rules formalize this for twin-engine aircraft, limiting how far they may fly from a suitable diversion airport. That constraint can push routes toward land even when the ocean looks shorter on the map.
The bottom line
Flight paths look curved because they are drawn on flat maps of a round Earth. The shortest route between two cities is a great circle arc, and the Mercator projection stretches that arc into a bow, especially near the poles where the route naturally swings.
On top of that geometry, jet streams, weather systems, and emergency planning bend real flights further, so today's path may differ from yesterday's. Next time the seat-back map arcs toward Greenland, you will know the curve is the shortcut, and the straight line was the illusion, a useful piece of aviation trivia alongside why runways carry numbers and why planes sometimes dump fuel before landing.
FAQ
Q: What is a great circle route in simple terms?
A: It is the shortest path between two points on a sphere: the arc you get by stretching a string tightly across a globe. Any circle centered on Earth's center, like the equator, is a great circle, and segments of such circles give the shortest flight routes.
Q: Why do flights from North America to Europe go over Greenland?
A: Because that arc is the great circle route, the shortest path on the sphere. On a flat Mercator map it looks like a detour north, but in three dimensions it covers less distance than the straight-looking line across the Atlantic.
Q: Do pilots ever fly straight lines on a map?
A: A constant-compass-heading route, called a rhumb line, draws straight on a Mercator map but is physically longer than the great circle. Pilots avoid it on long hauls because it wastes fuel and time.
Q: Can the jet stream really change a flight's route?
A: Yes. Jet streams regularly reach 80 to 140 mph and can exceed 200 mph, flowing mostly west to east. Planners bend routes to ride tailwinds or dodge headwinds, which is why westbound flights often take longer and fly different tracks than eastbound ones.
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Topics: flight paths, great circle routes, aviation science, map projections
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