Haversine Formula

haversine.js

// Haversine formula calculates the distance between two points (as the crow flies),
// taking into account the curvature of the earth.
// Returns a distance value in Kilometers.
function haversine(lat1, lon1, lat2, lon2) {
	var R = 6371; // km
	var dLat = (lat2-lat1).toRad();
	var dLon = (lon2-lon1).toRad();
	var lat1 = lat1.toRad();
	var lat2 = lat2.toRad();

	var a = Math.sin(dLat/2) * Math.sin(dLat/2) +
	        Math.sin(dLon/2) * Math.sin(dLon/2) * Math.cos(lat1) * Math.cos(lat2); 
	var c = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1-a)); 
	var d = R * c; // Kilometers

	return d;
}

// Converts numeric degrees to radians
if (typeof Number.prototype.toRad == 'undefined') {
  Number.prototype.toRad = function() {
    return this * Math.PI / 180;
  }
}