fn get_bearing(origin : & geo::Point<f64>, destination : & geo::Point<f64>) -> f64 {
// https://gist.github.com/geografa/1366401
        let (lon1, lat1) = ( origin.x().to_radians(), origin.y().to_radians() );
        let (lon2, lat2) = ( destination.x().to_radians(), destination.y().to_radians() );
        let dlon = lon2-lon1;

        let bearing = f64::atan2(f64::sin(dlon)*f64::cos(lat2),f64::cos(lat1)*f64::sin(lat2)-f64::sin(lat1)*f64::cos(lat2)*f64::cos(dlon));

        bearing.to_degrees()
}

S = cos φB sin ∆L
C = cos φA sin φB − sin φA cos φB cos ∆L
beta = atan2(S, C);