code: def light_time_difference(position, observer_position):
      
      # From 'pos1', form unit vector 'u1' in direction of star or light
      # source.
  
      dis = length_of(position)
      u1 = position / dis
  
      # Light-time returned is the projection of vector 'pos_obs' onto the
      # unit vector 'u1' (formed from 'pos1'), divided by the speed of light.
  
      diflt = einsum('a...,a...', u1, observer_position) / C_AUDAY
      return diflt
docs: Returns the difference in light-time, for a star,
        between the barycenter of the solar system and the observer (or
        the geocenter).