docs: def convert(lat, lon):
      """convert lat/lon from the ecliptic to the invariable plane."""
  
      x = numpy.cos(lon) * numpy.cos(lat)
      y = numpy.sin(lon) * numpy.cos(lat)
      z = numpy.sin(lat)
  
      # Invariable plane: values in arcseconds.
      epsilon = 5713.86
      omega = 387390.8
  
      coseps = numpy.cos(epsilon * secrad)
      sineps = numpy.sin(epsilon * secrad)
      cosom = numpy.cos(omega * secrad)
      sinom = numpy.sin(omega * secrad)
  
      xi = x * cosom + y * sinom
      yi = coseps * (-sinom * x + cosom * y) + sineps * z
      zi = - sineps * (-sinom * x + cosom * y) + coseps * z
  
      lat = numpy.arcsin(zi)
      lon = numpy.arctan2(yi, xi)
      return (lat, lon)