A brief history of pulsar time

Radio pulsars were discovered because their sources, spinning neutron stars, keep time with extraordinary precision. Indeed, some rival the very best atomic clocks. But they are not perfect clocks. Allowance must be made for their steady deceleration due to the action of an electromagnetic torque; and some undergo sudden, unpredictable increases in their frequencies, known as glitches. One of the best studied pulsars is PSR0531+21, more famous as the central pulsar of the Crab Nebula - the relic of a supernova explosion observed by Chinese astronomers in AD 1054. A large glitch on 29 August 1989 shortened this pulsar's 33-millisecond period by about 3 nanoseconds, a much bigger jump than in previous glitches. On page 706 of this issue, Lyne, Graham Smith and Pritchard present an analysis of the times of arrival of pulses from this pulsar which suggests a physical description of the interior of the neutron star that is both more complicated and more intriguing than any that has been seen so far.