code: def fraction_illuminated(ephemeris, body, t):
      
      a = phase_angle(ephemeris, body, t).radians
      return 0.5 * (1.0 + cos(a))
docs: Compute the illuminated fraction of a body viewed from Earth.
  
      The ``body`` should be an integer or string that can be looked up in
      the given ``ephemeris``, which will also be asked to provide
      positions for the Earth and Sun.  The return value will be a
      floating point number between zero and one.  This simple routine
      assumes that the body is a perfectly uniform sphere.