Have you ever looked at a flight path map and wondered why planes fly over Greenland to get from New York to London? The answer lies in spherical geometry — specifically, the great circle route.
What is a Great Circle?
A great circle is any circle drawn on a sphere whose centre coincides with the centre of the sphere. The equator is a great circle. Meridians of longitude are great circles. The shortest path between any two points on a sphere always follows a great circle arc — and that's what flight paths follow.
Why Does This Look "Wrong" on Maps?
Most world maps use the Mercator projection, which preserves angles but distorts areas and distances near the poles. A straight line on a Mercator map is a rhumb line (constant compass bearing), not a great circle. The great circle from New York to London curves north — over Newfoundland and near Greenland — because that truly is the shortest path on the globe, even though it looks longer on a flat map.
Try it yourself: hold a piece of string taut between New York and London on a globe. It will arc over Greenland.
The Haversine Formula
HowFarFrom uses the Haversine formula to calculate great circle distances from latitude/longitude coordinates. It accounts for the Earth's curvature and gives results accurate to within 0.3% for most distances.
The formula:
d = 2r · arcsin(√(sin²(Δlat/2) + cos(lat₁)·cos(lat₂)·sin²(Δlon/2)))Where r = 6,371 km (Earth's mean radius), Δlat and Δlon are the differences in latitude and longitude.
Why Don't Planes Always Fly Great Circle Routes?
Real flight paths deviate from pure great circles for several reasons: jet streams (pilots fly with them to save fuel), restricted airspace (overflying conflict zones or certain countries), ETOPS rules for twin-engine aircraft over oceans, and weather avoidance.
The published flight distance you see on booking sites is the great circle distance — but the actual route flown may be longer.