Jansky was born in Norman, Oklahoma, and studied at the University of Wisconsin where he received his Bachelor of Science degree in Physics in 1927.
In 1928 he joined the Bell Telephone Laboratories in Holmdel, New Jersey. Bell Labs wanted to investigate using "short waves" (wavelengths of about 10-20 meters) for transatlantic radio telephone service. Jansky was assigned the job of investigating the sources of static that might interfere with radio voice transmissions.
He built an antenna designed to receive radio waves at a frequency of 20.5 MHz (wavelength about 14.5 meters). It was mounted on a turntable that allowed it to rotate in any direction, earning it the name "Jansky's merry-go-round". By rotating the antenna, one could find what the direction was to any radio signal.
After recording signals from all directions for several months, Jansky identified three types of static: nearby thunderstorms, distant thunderstorms, and a faint steady hiss of unknown origin. Jansky spent over a year investigating the third type of static. It rose and fell once a day, leading Jansky to think at first that he was seeing radiation from the Sun. But after a few months of following the signal, the brightest point moved away from the position of the Sun. The signal repeated not every 24 hours, but every 23 hours and 56 minutes. This is characteristic of the fixed stars, and other objects far from our solar system (sidereal day). He eventually figured out that the radiation was coming from the Milky Way and was strongest in the direction of the center of our Milky Way galaxy, in the constellation of Sagittarius.
The discovery was widely publicized, appearing in the New York Times of May 5, 1933.
Jansky wanted to follow up on this discovery and investigate the radio waves from the Milky Way Galaxy in more detail. He proposed to Bell Labs to build a 30 meter diameter dish antenna. But Bell Labs 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.
Many scientists were fascinated by Jansky's discovery, but no one followed up on it for several years. It was during the Great Depression, and observatories could not afford take on any new projects.
Two men who learned of Jansky's discovery in 1933 were of great influence on the later development of the new study of radio astronomy: one was Grote Reber, who singlehandedly built a radio telescope in his back yard in 1937 and did the first systematic survey of radio waves from the sky. The second was John Kraus, who, after World War II, started a radio observatory at Ohio State University and wrote a textbook on radio astronomy, which is still the "bible" for radio astronomers.
In honor to Jansky, the unit used by radio astronomers for the strength (or flux density) of radio sources is the jansky (symbolic form, Jy).
Reference
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My Brother Karl Jansky and His Discovery of Radio Waves from Beyond the Earth