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The discovery

 

Three sources of scintillation of radio sources:
  terrestrial ionosphere
  interplanetary gas
  ionised interstellar medium

Studying interplanetary scintillation at Cambridge
Hewish and Bell [14] observed the recordings
shown in Figure 2, period = 1.337 seconds.

  human made?
  from a spacecraft?
  extraterrestrial civilisation?

  
Figure 2: Discovery observations of the first pulsar. (a) The first recording of PSR 1919+21; the signal resembled the radio interference also seen on this chart. (b) Fast chart recording showing individual pulses as downward deflections of the trace. From Lyne & Graham-Smith 1990 [23].