It became clear soon after their discovery that pulsars are
excellent celestial clocks. In the original discovery
paper [97], the period of the first pulsar to be discovered, PSR B1919+21,
was found to be stable to one part in
over a time-scale of a few months. Following the discovery of
the millisecond pulsar B1937+21 in 1982 [17] it was demonstrated that its period could be measured to one
part in
or better [65]. This unrivaled stability leads to a host of applications
including time keeping, probes of relativistic gravity and
natural gravitational wave detectors.