  We present an analysis of $23~$yr of pulse arrival times for PSR B1259$-$63.
The pulsar is in a binary orbit about its approximately $20~M_{\odot}$
companion LS$~$2883. Our best-fitting timing solution has none of the
pulse-number ambiguities that have plagued previous attempts to model the
binary orbit. We measure significant first and second time derivatives of the
projected semimajor axis and longitude of the periastron of the orbit. These
variations are found to be consistent with the precession of the orbital plane
due to classical spin-orbit coupling. The derived moment of inertia and spin of
the companion are consistent with the companion rotating at near-breakup
velocity. The system configuration is also consistent with the geometry derived
from both the polarisation of the radio emission and the eclipse of the pulsar
by the equatorial disc of the companion. We find strong evidence for orbital
period decay that can be attributed to mass loss from the companion star. We
also measure a significant proper motion that locates the birth of the system
in the Centaurus OB1 association. By combining proper motion of the pulsar with
radial velocity measurements of the companion, we measure the three-dimensional
velocity of the system. This velocity is used to constrain the masses of the
stars prior to the supernova explosion and the kick the pulsar received at or
immediately after the explosion.
