Great-Circle vs Rhumb-Line Navigation
Why the shortest route between two airports is a curved line on a flat map, how a great circle differs from a constant-heading rhumb line, and when the difference matters.
Open a route between two distant airports and the line looks curved, bending toward the pole. That curve is a great circle — and it really is the shortest path between the two points across the surface of the Earth.
What a great circle is
A great circle is any circle on the globe whose plane passes through the centre of the Earth — the equator and every line of longitude are great circles. The shorter arc of the great circle through two points is the shortest distance between them across the surface. That is the distance our distance & flight-time tool and the route pages report.
The catch: except along the equator or a meridian, the true course of a great circle changes continuously as you fly it. You leave on one bearing (the *initial bearing*) and arrive on a different one (the *final bearing*), because the meridians you cross are converging. That is why the tools show an initial bearing for each direction rather than a single course.
What a rhumb line is
A rhumb line (or loxodrome) is the opposite trade-off: a track of constant true course that crosses every meridian at the same angle. It is simple to fly — hold one heading — but except for north-south or equatorial tracks it is longer than the great circle, and the gap grows with distance and latitude.
Why the map lies
On the familiar Mercator projection, a rhumb line is a straight line and a great circle is a curve that bows toward the nearer pole. On a gnomonic chart it is the reverse: great circles are straight. Neither projection is "wrong" — they are flattening a sphere in different ways. The curved airline route on a flat map is simply a great circle drawn on a Mercator grid.
When the difference matters
- Short legs (a few hundred miles): the great-circle and rhumb-line distances and headings are nearly identical — flying a constant heading costs almost nothing.
- Long, high-latitude legs (transoceanic, polar): the great circle can be dramatically shorter, so airliners approximate it by stringing together waypoints and updating heading along the way; modern flight management systems fly the great circle directly.
In practice
Enter any two airports in the distance tool to see the great-circle distance and the initial bearing, or open a route page to compare a specific city pair. For real flight planning, remember the number is a great-circle *idealisation* — actual routings follow airways, procedures and ATC.
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*Reference and training only — not for navigation. Always consult the current AIM, your aircraft’s POH/AFM and a certificated flight instructor.*