Simple Haversine formula implementation to calculate geographic distance on earth having a pair of Latitude/Longitude points.
The haversine formula determines the great-circle distance between two points on a sphere given their longitudes and latitudes.
The formula in this library works on the basis of a spherical earth, which is accurate enough for most purposes.
Disclaimer: The earth is not quite a sphere. This means that errors from assuming spherical geometry might be considerable depending on the points, so: don't trust your life on this value.
Check this detailed information.