Returns the difference in light-time, for a star,
      between the barycenter of the solar system and the observer (or
      the geocenter).

def light_time_difference(position, observer_position):
    """Returns the difference in light-time, for a star,
      between the barycenter of the solar system and the observer (or
      the geocenter).

    """
    # 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