The following C++ snippet computes the arc (in radians) and the distance (in meters) between two positions on Earth using the law of haversines.

It assumes a Position class exists, with two public members for the longitude and the latitude (resp. "lon" and "lat").

/// @brief The usual PI/180 constant
static const double DEG_TO_RAD = 0.017453292519943295769236907684886;
/// @brief Earth's quatratic mean radius for WGS-84
static const double EARTH_RADIUS_IN_METERS = 6372797.560856;

/** @brief Computes the arc, in radian, between two WGS-84 positions.
*
* The result is equal to <code>Distance(from,to)/EARTH_RADIUS_IN_METERS</code>
*
* where:<ul>
*    <li>d is the distance in meters between 'from' and 'to' positions.</li>
*    <li>h is the haversine function: <code>h(x)=sin²(x/2)</code></li>
* </ul>
*
* The haversine formula gives:
*    <code>h(d/R) = h(from.lat-to.lat)+h(from.lon-to.lon)+cos(from.lat)*cos(to.lat)</code>
*
* @sa http://en.wikipedia.org/wiki/Law_of_haversines
*/
double ArcInRadians(const Position& from, const Position& to) {
double latitudeArc  = (from.lat - to.lat) * DEG_TO_RAD;
double longitudeArc = (from.lon - to.lon) * DEG_TO_RAD;
double latitudeH = sin(latitudeArc * 0.5);
latitudeH *= latitudeH;
double lontitudeH = sin(longitudeArc * 0.5);
lontitudeH *= lontitudeH;