text: function circleDistance(lon1, lat1, lon2, lat2) {
    var R = 6371; // Radius of the earth in km
    var dLat = (lat2-lat1).toRad();  // Javascript functions in radians
    var dLon = (lon2-lon1).toRad();
    var a = Math.sin(dLat/2) * Math.sin(dLat/2) +
            Math.cos(lat1.toRad()) * Math.cos(lat2.toRad()) *
            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;
  }
  
  /** Converts numeric degrees to radians */
  if (typeof(Number.prototype.toRad) === "undefined") {
    Number.prototype.toRad = function() {
      return this * Math.PI / 180;
    }
  }
  
  //How to use it in the HTML file.
  // window.navigator.geolocation.getCurrentPosition(function(pos) {
  //   console.log(pos);
  //   console.log(
  //     distance(pos.coords.longitude, pos.coords.latitude, 42.37, 71.03)
  //   );
  // });
  