= Hulse-Taylor timing experiment
{c}
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.<The Discovery of the Binary Pulsar by Russell Alan Hulse>[The Discovery of the Binary Pulsar, lecture delivered 8 December 1993, pp. 53–58 and 62–65, Figures 3–4 and 8–10]{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.<Discovery of a Pulsar in a Binary System by Russell Alan Hulse and Joseph Hooton Taylor Jr.>[L51–L52, section II, Tables 1–2, Figure 1 and equations 1–4]{ref} An orbit explains both signs of the apparent <period> change without requiring the <stellar rotation> to alternate between speeding up and slowing down.
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