Great Circle Distance Calculator

The great circle distance is the shortest distance between two points on a sphere (the surface of the Earth). A great circle is any circle on a sphere that has the same center and radius as the sphere. The shortest path between two points follows a great circle, not a straight line on a flat map.

The haversine formula calculates great circle distance from two lat/long pairs. It is numerically stable and avoids rounding errors. The formula uses: a = sin²(Δlat/2) + cos(lat1) * cos(lat2) * sin²(Δlon/2); then d = 2R * arcsin(sqrt(a)), where R is Earth's radius (6,371 km).

Maps project the 3D Earth onto a 2D surface, distorting distances and directions. On a flat map, the shortest path looks curved when viewed on a globe. Following a great circle (which looks curved on a Mercator map) is actually the shortest route. Airlines use great circle routes to minimize flight distances.

Latitude measures distance north or south of the equator, from 0 to 90 degrees. North latitudes are positive; south are negative. Longitude measures distance east or west of the Prime Meridian, from 0 to 180 degrees. East longitudes are positive; west are negative. Together they uniquely identify any location on Earth.

Earth is not a perfect sphere but an oblate spheroid (flattened at poles). The mean radius is approximately 6,371 km, used for most distance calculations. The equatorial radius is 6,378 km; the polar radius is 6,357 km. Different contexts may use slightly different values.