code: def _calculate_distance(latlon1, latlon2):
      
      lat1, lon1 = latlon1
      lat2, lon2 = latlon2
      dlon = lon2 - lon1
      dlat = lat2 - lat1
      R = 6371  # radius of the earth in kilometers
      a = np.sin(dlat / 2)**2 + np.cos(lat1) * np.cos(lat2) * (np.sin(dlon / 2))**2
      c = 2 * np.pi * R * np.arctan2(np.sqrt(a), np.sqrt(1 - a)) / 180
      return c
docs: Calculates the distance between two points on earth.