Twenty years of timing SS 433

We present observations of the optical "moving lines" in spectra of the Galactic relativistic jet source SS 433 spread over a 20 year baseline from 1979 to 1999. The redshift/blueshift of the lines reveal the apparent precession of the jet axis in SS 433, and we present a new determination of the precession parameters based on these data. We investigate the amplitude and nature of time- and phase-dependent deviations from the kinematic model for the jet precession, including an upper limit on any precessional period derivative of (P)over dto < 5 x 10(-5). We also discuss the implications of these results for the origins of the relativistic jets in SS 433.