August 6, 1967

PSR B1919+21

The first recorded trace of the signal later identified as pulsar CP 1919

The first pulsar entered the human record as a patch of "scruff" occupying roughly one ten-millionth of the chart paper.

A mark on chart paper

On 6 August 1967 a chart recorder attached to Cambridge's Four-Acre Array registered a signal from the radio source later designated CP 1919 and PSR B1919+21. An original chart carrying that date survives in the Antony Hewish papers.

Jocelyn Bell, the postgraduate student responsible for inspecting the survey charts, marked the anomalous trace for attention. The array had been built to study interplanetary scintillation, not to look for pulsars.

Its pulsed nature was not recognized that day.

The pulses came in November

The trace returned from the same fixed celestial position on later passes, which prompted Bell to retrieve earlier records and persuade her supervisor that this was not ordinary terrestrial interference.

The regular sequence of pulses roughly one and a third seconds apart was not obtained until a faster recording on 28 November 1967.

The "Little Green Men" label belongs to that later stage, after the signal's regularity had been established — it was not the annotation on the August chart.

Whose discovery, and when

Cambridge distinguishes the first detection on 6 August from the first observation of resolved pulses on 28 November, so calling August the discovery date holds only under the first definition. The surviving chart proves when the instrument recorded the signal, not the day Bell appreciated its importance; reconstructions place that recognition later in August.

The listing gives Hewish too direct a role on the day. He was Bell's supervisor and central to the investigation, but the artifact and Bell's own accounts credit her with inspecting and recognizing the anomaly.

Nothing was announced in 1967. Hewish, Bell, John Pilkington, Paul Scott and Robin Collins published in Nature on 24 February 1968.

Sources

Researched 24 Aug 2026 7 sources not yet audited Date disputed

How this was checked

Researched from the web into a fact sheet and rewritten from that sheet. This article has not yet had an independent model audit. How the pipeline works →

Date disputed. Sources disagree about when this happened.

What the sources leave uncertain

August 6 is solidly documented as the date on the surviving first trace, but sources disagree over whether to call that the “discovery.” Cambridge distinguishes the first detection on August 6 from the first observation of resolved pulses on November 28.
The surviving chart proves when the instrument recorded the signal, not necessarily the exact day Bell first appreciated its importance. Historical reconstructions place her recognition of its recurrence later in August.
The event's listed wording gives Hewish too direct a role on August 6. He was Bell's supervisor and central to the investigation, but the contemporary artifact and Bell's accounts credit her with inspecting and recognizing the chart anomaly.