Geodetic precession in PSR J1141-6545

We present observations that show dramatic evolution of the mean pulse profile of the relativistic binary pulsar J1141 - 6545 over a period of 5 yr. This is consistent with the precession of the pulsar spin axis due to relativistic spin-orbit coupling. Observations made between 1999 and 2004 with a number of instruments at the Parkes radio telescope demonstrate a steady, secular evolution of the mean total intensity profile, which increases in width by more than 50% during the 5 yr period. Analysis of the changing position angle of the linearly polarized component of themean profile suggests that our line of sight is shifting closer to the core of the emission cone. We find that the slope of the position angle swing across the center of the pulse steepens with time and use a simplified version of the rotating vector model to constrain the magnitude and direction of the change in our line-of-sight angle relative to the pulsar magnetic axis. The fact that we appear to be moving deeper into the emission cone is consistent with the non-detection of this pulsar in previous surveys.