code: function (other) { // (LatLng) -> Number
  		other = L.latLng(other);
  
  		var R = 6378137, // earth radius in meters
  		    d2r = L.LatLng.DEG_TO_RAD,
  		    dLat = (other.lat - this.lat) * d2r,
  		    dLon = (other.lng - this.lng) * d2r,
  		    lat1 = this.lat * d2r,
  		    lat2 = other.lat * d2r,
  		    sin1 = Math.sin(dLat / 2),
  		    sin2 = Math.sin(dLon / 2);
  
  		var a = sin1 * sin1 + sin2 * sin2 * Math.cos(lat1) * Math.cos(lat2);
  
  		return R * 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1 - a));
  	}
docs: Haversine distance formula, see http://en.wikipedia.org/wiki/Haversine_formula TODO move to projection code, LatLng shouldn't know about Earth