The Wow! Signal — August 15, 1977

Ohio State University's Big Ear was a stationary radio telescope. It did not track a target across the sky. Instead, Earth's rotation carried each part of the sky through the telescope's narrow listening beam.

That created a built-in test. A signal from a fixed position in the sky should gradually strengthen as the source entered the beam, peak near the center, and weaken as it moved out again. The complete passage would take seventy-two seconds.

On August 15, 1977, during a search for possible extraterrestrial radio signals, Big Ear recorded a powerful signal concentrated in a very narrow range of frequencies. That means most of its radio energy arrived inside one small part of the spectrum, the kind of concentrated signal SETI surveys were designed to notice. It rose and fell across the full seventy-two seconds in the shape expected from something fixed in the sky.

The telescope's computer encoded the changing strength as the characters 6EQUJ5. The sequence was not a message. It was a compact way of printing six successive intensity measurements. When astronomer Jerry Ehman later reviewed the paper output, he circled the sequence and wrote "Wow!" beside it.

The telescope's observing method made the event look consistent with a source passing through its beam. But the signal was never detected again. Without a second observation, astronomers could not refine its position, compare its behavior, or test an explanation against a repeat.

Proposals have included terrestrial interference, reflected human transmissions, and natural emission associated with hydrogen. None has become a generally accepted identification of the original signal.