Pulsars are excellent celestial clocks. The period of the first pulsar  was found to be stable to one part in 107 over a few months. Following the discovery of the millisecond pulsar B1937+21  it was demonstrated that its period could be measured to one part in 1013 or better . This unrivaled stability leads to a host of applications including uses as time keepers, probes of relativistic gravity and natural gravitational wave detectors.
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