code: public static final double speed(long time1, double lat1, double lon1, long time2, double lat2, double lon2)
      {
          checkLatitude(lat1);
          checkLongitude(lon1);
          checkLatitude(lat2);
          checkLongitude(lon2);
          double distance = distance(lat1, lon1, lat2, lon2);
          double duration = time2-time1;
          double hours = duration/3600000.0;
          double speed = distance/hours;
          return speed;
      }
docs: Returns the speed needed to move from (time1, lat1, lon1) to (time2, lat2, lon2)
  @param time1
  @param lat1
  @param lon1
  @param time2
  @param lat2
  @param lon2
  @return Knots