Great-circle distance: why flight paths curve on maps

What a great circle is

Slice a globe through its centre and the cut traces a great circle: the equator is one, and so is every line of longitude paired with its opposite. Through any two points that aren’t exactly opposite each other there is one great circle, and the shorter arc of it is the shortest path between them over the surface.

That is why long-haul routes on a globe arc toward the poles: heading north-east first and curving round is shorter than holding one compass direction.

Why the shortest route looks curved on a map

Every flat map has to stretch or squeeze the globe. On the familiar Mercator map, lines of constant compass heading are straight, and areas far from the equator are enlarged, so a great-circle route bends away from the straight line. On our own world maps, which use the Equal Earth projection, the shapes are distorted differently, but a great circle still looks curved.

How much longer a straight map line is

A line that keeps one compass heading is called a rhumb line; on a Mercator map it is straight. We calculated both distances for some long airport pairs, on the same sphere, so the difference comes only from the shape of the path.^{[1–2] sources}

The gap grows on long routes at high latitudes: in this set it is largest for DXB–LAX, where the constant-heading line is 25% longer. Along the equator or along a line of longitude, the two paths are the same.^{[1–2] sources}

Why the direction out isn’t the reverse of the direction back

On a great circle the compass direction keeps changing. Leaving JFK for LHR, the route starts at a bearing of about 51° (NE); leaving LHR for JFK, it starts at about 288° (WNW) — not the exact opposite of the outbound direction. Our airport and route pages call this the initial bearing (glossary).^{[1–2] sources}

How we calculate distances

• Each airport is the single reference point OurAirports records for it — not a particular runway or terminal.
• We use the haversine formula on a sphere with the mean Earth radius of 6,371.0088 km.
• Treating the Earth as a sphere changes distances by less than 0.5% compared with the more exact ellipsoid — far less than the difference between the shortest path and a real flight path.
• Miles are statute miles; nautical miles are shown where navigation units help.^{[1–2] sources}

The full method is on our Data and methodology page.

What a flight distance doesn’t tell you

• The distance an aircraft flies, which follows its planned route rather than the geometric minimum.
• The flight time, which also depends on aircraft, winds and the route flown.
• Whether a nonstop flight exists between the two airports.
• Anything about the fare.

Treat great-circle distance as a consistent yardstick for comparing options — for example, which of two airports is closer to your destination — not as a schedule.

Try it yourself

• The Flight Distance Calculator gives the great-circle distance and initial bearing between any two airports.
• Route guides, such as the New York to London route guide, show the distances between every airport pair on a route.
• Choosing between airports? Distance is one input — see how to choose between nearby airports.