(t) and
      O(t) represent the positions of the target and observer
      relative to the solar system barycenter at time t; then `lt' is
      the solution of the light-time equation

                | T(et-lt) - O(et) |
         lt = ------------------------                            (1)
                         c

      The ratio

          | T(et) - O(et) |
        ---------------------                                     (2)
                  c

      is used as a first approximation to `lt'; insert