Jansky's Hiss
Later explainedSummary
From 1931 to 1933 Karl Jansky of Bell Laboratories traced a steady radio hiss to the Milky Way. It was the first radio emission detected from space.
He had been asked to find the sources of static on transatlantic radiotelephone links, not to study the sky. The hiss kept time with the stars rather than the Sun, and it was strongest toward Sagittarius, in the direction of the galaxy's centre. Bell Laboratories had its answer about static and moved him to other work. The hiss is now understood as mostly radiation from fast electrons moving through the galaxy's magnetic field.
What is documented
The assignment. Jansky joined Bell Telephone Laboratories in 1928 and worked at its site in Holmdel, New Jersey. His task was to identify sources of static that might interfere with radio voice transmissions across the Atlantic.
The antenna. He built a directional antenna about 30 metres long that rolled round a circular track on four wheels taken from a Ford Model T, so that it could be pointed in any direction. It was known as Jansky's merry-go-round. It worked at 20.5 megahertz, a wavelength of about 14.6 metres. John Kraus writes that he set it turning and his recorder running continuously in the late summer of 1931.
Three kinds of static. Jansky sorted what he recorded into static from nearby thunderstorms, static from distant thunderstorms, and a third kind: in the words of the National Radio Astronomy Observatory's account, "a faint steady hiss of unknown origin". In a letter to Nature he called it "hiss type atmospherics".
Sun time or star time. The hiss rose and fell once a day, and at first Jansky thought he was seeing radiation from the Sun. He spent more than a year on it. The peak came round every 23 hours and 56 minutes. That is the length of the sidereal day, the time the Earth takes to turn once relative to the stars, and four minutes shorter than the solar day. By the clock, the peak arrives about four minutes earlier each day, and the difference comes to about two hours after a month, about six after three months, and a full day after a year. A source that behaves this way is fixed among the stars. The hiss was coming from the Milky Way, and it was strongest in the direction of the galaxy's centre, in the constellation Sagittarius.
Publication. Jansky reported his directional studies of static, the hiss among them, in the Proceedings of the Institute of Radio Engineers in December 1932. His letter "Radio Waves from Outside the Solar System" appeared in Nature on 8 July 1933, and his paper "Electrical Disturbances Apparently of Extraterrestrial Origin" in the Proceedings in October 1933. The discovery was reported in The New York Times on 5 May 1933, and an NBC radio programme was billed as "radio sounds from among the stars".
What followed. Jansky proposed building a dish antenna 30 metres across. Bell Laboratories declined. As the observatory's account puts it, the company "had the answer they wanted about static: the static was not a problem for transatlantic radio communication". Jansky was assigned to another project and did no more radio astronomy. Kraus records that when he presented his final paper on the discovery, on 3 July 1935 at a convention of the Institute of Radio Engineers in Detroit, "scarcely two dozen people were in the audience". Kraus's summary of the decade: "astronomers and engineers hardly raised an eyebrow." Jansky died in 1950, aged 44.
Reber. In 1937 Grote Reber single-handedly built a radio telescope in his back yard, and with it made the first systematic survey of radio waves from the sky.
The legacy. Radio astronomers measure the strength of radio sources in janskys: one jansky is 10⁻²⁶ watts per square metre per hertz. A full-scale replica of the merry-go-round stands at Green Bank, West Virginia.
Leading explanations
Radio emission from the Milky Way. Established. This was Jansky's own conclusion, drawn from the sidereal period and the direction of the peak, and it has stood since 1933.
Synchrotron radiation from cosmic-ray electrons. Established. Stars are not the source. Astronomers had correctly deduced, in the words of the radio astronomy textbook by James Condon and Scott Ransom, that stars "would be undetectably faint as radio sources". The same text states that "cosmic-ray electrons in the interstellar magnetic field emit the synchrotron radiation that accounts for most of the continuum emission from our Galaxy at frequencies below about 30 GHz". Jansky's 20.5 megahertz lies far below that.
The Sun. Rejected. This was Jansky's first idea. A source tied to the Sun peaks at the same clock time every day. The hiss did not.
Thunderstorms. Rejected. Jansky separated the hiss from the static of both nearby and distant storms.
What the popular version gets wrong
"Jansky was hearing the stars." That is how the discovery was presented in 1933: the NBC programme spoke of radio impulses "from somewhere among the stars", and Jansky himself used the term "star static". Ordinary stars are far too faint at these wavelengths to be detected. What Jansky recorded is mostly the radiation of fast electrons spiralling in the magnetic field that threads the space between the stars.
"The hiss came from the centre of the galaxy." It came from the Milky Way at large, and it was strongest toward Sagittarius, in the direction of the centre. The compact radio source at the galaxy's nucleus was not detected until 1974, when Bruce Balick and Robert Brown found it with an interferometer at the National Radio Astronomy Observatory. It was unresolved at a tenth of a second of arc.
Current status
Later explained. The hiss is the radio glow of the Milky Way, dominated at Jansky's frequency by synchrotron radiation. His identification of where it came from, made with a rotating antenna and a recorder, has not needed correction.
The registry keeps the record because it is the founding anomaly of radio astronomy, and because of how little happened next. A telephone company asked a question about static, got its answer, and moved its engineer to other work. The first systematic survey of the radio sky was made by one man, with a telescope he built in his own back yard in 1937.
Sources
- Jansky, K. G. (1932). "Directional Studies of Atmospherics at High Frequencies." Proceedings of the Institute of Radio Engineers 20, 1920 (December 1932).
- Jansky, K. G. (1933). "Radio Waves from Outside the Solar System." Nature 132, 66 (8 July 1933).
- Jansky, K. G. (1933). "Electrical Disturbances Apparently of Extraterrestrial Origin." Proceedings of the Institute of Radio Engineers 21, 1387 (October 1933).
- Ghigo, F. (2003). "Karl Jansky and the Discovery of Cosmic Radio Waves." National Radio Astronomy Observatory, Green Bank.
- Kraus, J. (1981). "The First 50 Years of Radio Astronomy, Part 1: Karl Jansky and His Discovery of Radio Waves from Our Galaxy." Cosmic Search 3(4), issue 12.
- Condon, J. J. and Ransom, S. M. Essential Radio Astronomy. National Radio Astronomy Observatory online text, chapters 1 and 5.
- Balick, B. and Brown, R. L. (1974). "Intense Sub-Arcsecond Structure in the Galactic Center." The Astrophysical Journal 194, 265 to 270.
Last reviewed: September 2026. Records are provisional. Where the evidence changes, the entry changes. Found an error? Tell us.