The radio saw the flare first

At 18:30 GMT on August 25, 1936, shortwave radio signals abruptly disappeared across much of the Western Hemisphere. Operators reported the fade-out from New York to Honolulu and along the west coast of South America. Long-wave service continued.

At Mount Wilson Observatory, the Sun looked eventful—but only later. A hydrogen spectroheliogram taken at 18:58 showed a brilliant eruption around a sunspot. Images taken every four minutes then recorded it fading until the region appeared normal at 19:22. There had been no solar observations during the preceding hour.

Astronomer R. S. Richardson therefore had an awkward sequence of evidence. The communications failure had a beginning; the astronomical record did not. In his short report, he carefully declined to declare the events simultaneous. But because the eruption was already declining in the first image, he suggested that it might have begun at 18:30—the time the radios went quiet.

The mechanism is now familiar. Radiation from a solar flare rapidly increases ionization in the lower ionosphere. High-frequency signals that would normally travel long distances by returning from higher layers are instead absorbed. NOAA describes the result as a radio blackout on Earth’s sunlit side.

The radio network was the victim of the flare, but it was also the better clock. Mount Wilson supplied the image and the plausible cause. Commercial operators, distributed across thousands of kilometres and trying to do something else, supplied the onset and geographical extent.

For about half an hour, the eruption existed more clearly in failed messages than in astronomy’s pictures.