Karl Jansky (1933): The Discovery of Radio Waves From Space

Karl Jansky discovered radio waves from space while hunting static for Bell Telephone Laboratories. In his 1933 paper he traced a steady hiss, which moved across the sky with the stars rather than the Sun, to the direction of Sagittarius and the center of the Milky Way. Radio astronomy began.

The question

Jansky was not looking for the universe. Bell Labs asked him to identify sources of static that could interfere with transatlantic radio-telephone links, a practical engineering problem. Until then, everything astronomers knew about the cosmos came through the narrow window of visible light, the only one telescopes had ever used. Nobody expected the sky itself to be a source of radio noise.

What the paper found

• Thunderstorms, as expected. Some of the static came from thunderstorms.
• A steady hiss. Besides the storms, there was a faint, constant hiss of unknown origin.
• It kept star time. The direction of the hiss shifted through the day in step with the stars rather than the Sun, the sign of a source fixed in space beyond the Solar System.
• It came from the galaxy’s center. Jansky traced it to the direction of Sagittarius, toward the center of the Milky Way.
• A first. This was the first detection of radio waves from space.

How he did it

At Holmdel, New Jersey, Jansky built a directional antenna that could rotate, nicknamed the “merry-go-round.” It worked at 20.5 MHz, a wavelength of about 14.6 meters. Because the antenna listened best in one direction, turning it let him measure where each kind of static came from. One of the few surviving records, scans from 16 September 1932 published in this paper, shows the noise rising each time the antenna’s beam swept across the Milky Way, about three times an hour.

At first he thought the hiss tracked the Sun. Over the following months its direction drifted away from the Sun, and by early 1933 he had concluded that it came from a direction fixed among the stars. The work took about three years of careful logging.

Why it mattered

Jansky’s report founded radio astronomy, the field that later discovered quasars, pulsars, found by Jocelyn Bell and Antony Hewish in 1967, and the cosmic microwave background, picked up in 1965 by another Bell Labs antenna at Holmdel. Radio dishes linked across the planet later produced the first image of a black hole’s shadow. The New York Times reported Jansky’s find on 5 May 1933. It is a reminder that big discoveries often begin as engineering problems.

What we know now

Jansky’s name is now a unit. In 1973 the International Astronomical Union adopted the jansky (Jy) as the unit of radio flux density, equal to 10^-26 watts per square meter per hertz.

Radio astronomy has grown into some of the largest instruments on Earth. The 500-meter FAST telescope in China, the world’s largest single-dish radio telescope, found more than 1,300 pulsars in its first decade (as of September 2026).

Who wrote it

Karl G. Jansky was a radio engineer at Bell Labs and the paper’s sole author. After the discovery he had little chance to continue the research, as other Bell Labs priorities took over. He was nominated for the Nobel Prize in Physics in 1949, before the importance of his work was widely appreciated.

Read the original

Jansky, K. G. (1933). “Electrical Disturbances Apparently of Extraterrestrial Origin.” Proceedings of the IRE 21, 1387-1398. doi:10.1109/JRPROC.1933.227458

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