MILLISECOND PULSAR PSR-1937+21 - A HIGHLY STABLE CLOCK

The stable rotation and sharp radio pulses of PSR 1937 plus 21 make this pulsar a clock whose long-term frequency stability approaches and may exceed that of the best atomic clocks. Improvements in measurement techniques now permit pulse arrival times to be determined in 1 hour at the Arecibo radio telescope with uncertainties of about 300 nanoseconds relative to atomic time. Measurements taken approximately every 2 weeks since November 1982 yield estimates of fractional frequency stability that continue to improve with increasing averaging time. The pulsar's frequency stability is at least as good as 6 multiplied by 10** minus **1**4 for averaging times longer than 4 months, and over the longest intervals the measurements appear to be limited by the stability of the reference atomic clocks.