This link might be helpful to you, as it details the use of the Haversine formula to calculate the distance.
Excerpt:
This script [in Javascript] calculates great-circle distances between the two points – that is, the shortest distance over the earth’s surface – using the ‘Haversine’ formula.
function getDistanceFromLatLonInKm(lat1,lon1,lat2,lon2) { var R = 6371; // Radius of the earth in km var dLat = deg2rad(lat2-lat1); // deg2rad below var dLon = deg2rad(lon2-lon1); var a = Math.sin(dLat/2) * Math.sin(dLat/2) + Math.cos(deg2rad(lat1)) * Math.cos(deg2rad(lat2)) * Math.sin(dLon/2) * Math.sin(dLon/2) ; var c = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1-a)); var d = R * c; // Distance in km return d; } function deg2rad(deg) { return deg * (Math.PI/180) }