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The surprise was a pulsar whose apparent period would not stay fixed. Russell Alan Hulse's Arecibo search used a multichannel receiver, pulsar dedispersion and computer searches over period and pulse width. Follow-up at higher time resolution retained the changing period, excluding inadequate sampling as its explanation. The observed variation exceeded the expected terrestrial Doppler shift; a predicted reversal from decreasing to increasing period appeared on 16 September 1974.^[ref]

The 1975 analysis fitted an eccentric Kepler orbit to Doppler shifts inferred from about 200 five-minute period measurements over ten days. That discovery fit was not yet the later precision arrival-time experiment.^[ref] An orbit explains both signs of the apparent period change without requiring the stellar rotation to alternate between speeding up and slowing down.

Russell Alan Hulse discovered a pulsar whose measured pulse period changed systematically during repeated observations. With Joseph Hooton Taylor Jr., he interpreted the variations as orbital Doppler shifts in a compact binary star system.

The changing period was the clue rather than a defect in the clock. A regular intrinsic pulse train observed from an orbiting source provides a radial velocity curve and orbital timing information. The discovery created a laboratory for later tests of relativistic gravity, including the loss of orbital energy through gravitational radiation.^[ref][ref]