code: def get_cartesian(lat, lon):
      
      R = 6371000.0  # metres
      x = R * math.cos(lat) * math.cos(lon)
      y = R * math.cos(lat) * math.sin(lon)
      z = R * math.sin(lat)
      return [x, y, z]
docs: the x-axis goes through long,lat (0,0), so longitude 0 meets the equator;
      the y-axis goes through (0,90);
      and the z-axis goes through the poles.
  
      In other words:
      * (0, 0, 0) is the center of earth
      * (0, 0, R) is the north pole
      * (0, 0, -R) is the south pole
      * (R, 0, 0) is somewhere in africa?
  
      :param lat: latitude in radians
      :param lon: longitude in radians