Jansky’s Experiments Lead to Radio Astronomy

An antenna set up to determine the causes of interference with radio transmission detected the first radio signals recognized as coming from outside the solar system.

Summary of Event

In 1928, Karl G. Jansky was hired as a radio engineer by Bell Telephone Laboratories. Bell Telephone Laboratories His first assignment was to investigate the causes of interference with transatlantic radiotelephone transmissions. This investigation required a sensitive antenna whose frequency response and sensitivity were very stable—ideal characteristics of any radio telescope. The device Jansky built, which was called a “Bruce array,” Bruce array consisted of two parallel frameworks of brass tubing; one frame was connected to a receiver, and the other acted as a signal reflector. The antenna was mounted on four wheels from a Model T Ford and rotated every twenty minutes on a circular track. The antenna, which was about 66 feet (20 meters) long and 13 feet (4 meters) in width and height, was nicknamed the “merry-go-round.” [kw]Jansky’s Experiments Lead to Radio Astronomy (1930-1932)[Janskys Experiments Lead to Radio Astronomy (1930 1932)]
[kw]Radio Astronomy, Jansky’s Experiments Lead to (1930-1932)
[kw]Astronomy, Jansky’s Experiments Lead to Radio (1930-1932)
Radio astronomy
Radio telescopes
[g]United States;1930-1932: Jansky’s Experiments Lead to Radio Astronomy[07510]
[c]Science and technology;1930-1932: Jansky’s Experiments Lead to Radio Astronomy[07510]
[c]Astronomy;1930-1932: Jansky’s Experiments Lead to Radio Astronomy[07510]
Jansky, Karl G.
Skellett, Albert Melvin
Reber, Grote

The Goldstone Deep Space Communication Complex in the Mojave Desert of California, one of three complexes that make up the National Aeronautics and Space Administration’s Deep Space Network (DSN). The DSN provides radio communications for all of NASA’s interplanetary spacecraft and is also utilized for radio astronomy and radar observations of the solar system and the universe.


Jansky discovered that his instrument could detect three kinds of signals. Nearby thunderstorms created infrequent but powerful radio bursts. Distant thunderstorms created weak but steady signals as their radio signals were reflected off the ionosphere, Ionosphere an electrically conducting layer in the upper atmosphere. The third signal, which created a steady hiss in receivers, was at first a mystery. Even though this signal was not a serious problem for radio reception, Jansky continued his efforts to identify the source. The signal varied in intensity in a daily cycle, and Jansky initially suspected that it might originate with the Sun. The problem with this theory was that the signals reached their highest point a few minutes earlier each day.

Jansky, who was unfamiliar with astronomy, did not appreciate the significance of this observation, but a friend of his, Albert Melvin Skellett, did. The planet Earth takes 23 hours and 56 minutes to complete one rotation with respect to the stars. Because Earth moves in its orbit by about one degree per day, it takes an extra 4 minutes to complete a rotation with respect to the Sun. The signals were following sidereal (star) time; that is, they came from a source that was fixed with respect to the stars. In 1933, after a full year of observations, Jansky published his estimate of the source’s location: in the southern part of the Milky Way Milky Way galaxy galaxy in the direction of Sagittarius. In 1935, after additional analysis, Jansky reported that signals originated from all along the Milky Way.

Once Jansky understood the nature of the cosmic signals, he found that he was completely unable to detect the Sun, which he found quite puzzling. Jansky happened to be observing at a time of minimal sunspot Sunspots activity. If he had observed at a time of great sunspot activity (at “sunspot maximum”), his equipment should have detected solar radio emissions. Had he observed at sunspot maximum, however, the upper atmosphere would have been nearly opaque at the wavelengths he studied, and he probably would not have detected radio waves from the Milky Way. Jansky realized that if he could not detect the Sun, the signals from the Milky Way were not likely to originate in the stars. He suggested that the radio signals originated from interstellar dust and gas instead, a suspicion that has proved to be correct.

Jansky’s observations were described in a front-page article in The New York Times on May 5, 1933, and a national radio program broadcast a few seconds of cosmic radio noise. Nevertheless, the discovery had little significance for practical communications. Jansky proposed the construction of a 98-foot (30-meter) dish antenna to study the cosmic signals in greater detail, but his employers, believing that such investigations were more appropriate for academic researchers, turned down the proposal. Jansky went on to other areas of communications research and received a commendation for his work on radio direction finders during World War II. He had always been in poor health, and he died in 1950 at the age of forty-four, just as radio astronomy was beginning to flourish.

One of the few people who had sufficient knowledge of both astronomy and radio to take advantage of Jansky’s work was Grote Reber, who realized that investigating celestial radio sources would require completely different equipment from that Jansky had used. In 1937, Reber built a parabolic reflecting antenna with a diameter of 32.8 feet (10 meters), which was used to make maps of the sky by aiming the parabolic dish at different elevations and letting Earth’s rotation sweep the antenna across the field of view.


The discovery of cosmic radio signals led to the field of radio astronomy—the first time astronomers used any part of the electromagnetic spectrum other than the range of frequencies containing visible and infrared light. This new tool allowed astronomers for the first time to investigate the universe without having to depend on optical telescopes: Because radio waves penetrate cosmic dust and gas clouds, which block visible light, these radio waves could be used to map the structure of the Milky Way galaxy. In the years after Jansky and Reber’s work, radio astronomy discovered great explosive bursts in other galaxies, some of which emitted so much energy that their causes became the focus of scientific investigation. Radio astronomy also discovered pulsars as well as the faint background radiation that most astronomers consider to be the echo of the big bang. Astronomers were unprepared for the discovery that the universe could look so different at radio wavelengths.

Perhaps the most important effect of radio astronomy was to teach astronomers that every part of the electromagnetic spectrum reveals new phenomena and new types of celestial objects. The result was the opening up of new areas of investigation, including X-ray and ultraviolet astronomy. Radio astronomy
Radio telescopes

Further Reading

  • Burke, Bernard F., and Francis Graham-Smith. An Introduction to Radio Astronomy. 2d ed. New York: Cambridge University Press, 2002. Authoritative graduate-level text provides an introduction to radio telescopes as well as an overview of radio astronomy. Includes references, index, and an appendix on the origins of radio astronomy.
  • Hey, J. S. The Evolution of Radio Astronomy. New York: Science History Publications, 1973. History of radio astronomy concentrates mostly on the period after World War II but also provides some details regarding Jansky’s work.
  • Reber, Grote. “Radio Astronomy.” Scientific American 181 (September, 1949): 34-41. Written by the first radio astronomer at the dawn of radio astronomy, this article shows some of the first published radio maps of the heavens and makes a number of conjectures (later proven correct) about the causes of some cosmic radio signals.
  • Spradley, Joseph L. “The First True Radio Telescope.” Sky and Telescope 76 (July, 1988): 28-30. An account of the telescope of Grote Reber, who for more than a decade was the only scientist to pursue Jansky’s discoveries. Clearly describes Reber’s construction and early attempts to detect cosmic radio signals and reproduces some of his first radio maps of the heavens.
  • Sullivan, Woodruff T., III. “A New Look at Karl Jansky’s Original Data.” Sky and Telescope 56 (August, 1978): 101-105. Summarizes the events that led to Jansky’s discovery of extraterrestrial radio sources. Includes a reproduction of Jansky’s original chart data and a reexamination of his data in the light of modern discoveries. The language is moderately technical, appropriate to an introductory college science course.
  • _______. “Radio Astronomy’s Golden Anniversary.” Sky and Telescope 64 (December, 1982): 544-550. A pictorial review of early radio astronomy from Jansky’s initial discovery through 1967. Illustrations include photos of major historical figures as well as depictions of equipment and significant data recordings.
  • _______, ed. The Early Years of Radio Astronomy: Reflections Fifty Years After Jansky’s Discovery. 1984. Reprint. New York: Cambridge University Press, 2005. Collected recollections of many of the pioneers in the field place the birth and early era of radio astronomy in societal and scientific context. Includes many historical photographs.
  • Verschuur, Gerrit L. The Invisible Universe Revealed: The Story of Radio Astronomy. 2d ed. New York: Springer-Verlag, 1987. Written by a leading radio astronomer, this book summarizes the history of radio astronomy and surveys the major types of celestial radio phenomena.

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