Other meanings of Karl Jansky
Physics & Astronomy
Karl Guthe Jansky (1905–1950) was an American physicist and radio engineer who, in 1932, discovered radio waves emanating from the Milky Way, founding the field of radio astronomy. Working at Bell Telephone Laboratories, he built a directional antenna to investigate static that interfered with transatlantic radio telephone service. His detection of a faint, steady hiss that rose and fell daily and originated from the center of the galaxy was published in 1933, but the astronomical community largely ignored it until after World War II. Jansky's discovery is commemorated by the unit of radio flux density named after him, the jansky (Jy).
Karl Jansky was born on October 22, 1905, in Norman, Oklahoma, to Nellie Moreau and Cyril M. Jansky, an electrical engineer and dean of engineering at the University of Oklahoma. He earned a bachelor's degree in physics from the University of Oklahoma in 1927 and then joined Bell Telephone Laboratories in Holmdel, New Jersey, where he worked on radio interference problems.
His older brother, Cyril Jansky Jr., also became a radio engineer and co-founded the broadcasting engineering firm Jansky and Bailey. Karl's early work at Bell Labs focused on the directional characteristics of shortwave radio reception, which led him to construct a specialized antenna to measure static from all directions.1
In 1932, Jansky built a rotating antenna array, 30 meters long and operating at 20.5 MHz (14.6 m wavelength), mounted on a turntable to measure the direction of incoming static. He identified three types of static: local thunderstorms, distant thunderstorms, and a faint, steady hiss of unknown origin. The hiss rose and fell once a day, with a period of 23 hours and 56 minutes—the sidereal day—indicating that the source was fixed among the stars, not the Sun.
By 1933, Jansky concluded that the radiation originated from the Milky Way, with the strongest signal coming from the direction of the constellation Sagittarius, the galactic center. He published his findings in the Proceedings of the Institute of Radio Engineers in 1933 and 1935, but his work attracted little attention from astronomers, who lacked the tools and theoretical framework to pursue it.2
Jansky's discovery was largely ignored for nearly a decade. In 1937, Grote Reber, an amateur radio enthusiast, built a parabolic radio telescope in his backyard in Wheaton, Illinois, and confirmed Jansky's findings, mapping the sky at higher frequencies. Reber's work, published in 1940, revived interest in cosmic radio emission and is often credited with establishing radio astronomy as a scientific discipline.
Jansky himself did not pursue the astronomical implications further; he continued at Bell Labs, working on other radio engineering problems. He died of a heart attack in 1950 at age 44. In 1973, the International Astronomical Union named the unit of radio flux density the jansky (symbol Jy) in his honor. The Karl G. Jansky Very Large Array (VLA) in New Mexico, dedicated in 1980, also bears his name.3
Jansky's original antenna was dismantled in the late 1930s, and its location in Holmdel, New Jersey, is marked by a historical plaque. A replica, built in 2012, stands at the site of the National Radio Astronomy Observatory in Green Bank, West Virginia.
Jansky's discovery was initially reported in the New York Times in May 1933, but the article focused on the interference problem rather than the astronomical significance. Jansky himself was modest about his work, once remarking that he was "not an astronomer" and that the discovery was "a by-product of a search for the cause of static."
His early death at 44 meant he never saw the full flowering of radio astronomy, but his name endures in the jansky unit and in the VLA, which remains one of the most productive radio observatories in the world.
Karl Jansky's discovery is often cited as the accidental beginning of radio astronomy, a field that has since revealed the cosmic microwave background, pulsars, and quasars.
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