High-accuracy timing and positional astronomy : lectures held at the Astrophysics School IV, organized by the European Astrophysics Doctoral Network (EADN) in Graz, Austria, 19-31 August 1991 / J. van Paradijs, H.M. Maitzen, eds ; : us, : gw. -- Springer, c1993. -- (Lecture notes in physics ; 418).