Karl Guthe Jansky (October 22, 1905 –
February 14, 1950), was an American physicist and radio
engineer who in August 1931 first discovered radio waves emanating from the Milky Way. He is considered one of the
founding figures of radio astronomy.
Early life
Karl Guthe Jansky was born in what was then the Territory of Oklahoma where his
father, Cyril M. Jansky, was Dean of the college of Engineering at the University of
Oklahoma at Norman. Dean Jansky, born in Wisconsin of Czech immigrants, had started teaching at the age of sixteen. He was
a teacher throughout his active life, retiring as professor of Electrical Engineering at the University of Wisconsin. Karl
Jansky's mother, née Nellie Moreau, was of French and English descent.
He was named after Dr. Karl Guthe, who had been an important mentor to Karl's father, C. M. Jansky, who was an engineer with a
strong interest in physics, a trait passed on to his sons. Karl's brother Cyril Jansky Jr., who was ten years older, helped build
some of the earliest radio transmitters in the country, including 9XM in Wisconsin (now
WHA of Wisconsin Public Radio) and 9XI in
Minnesota (now KUOM).
Education and engineering
Jansky attended college at the University of Wisconsin-Madison where
he received his BS in physics in 1927. In 1928 he joined the Bell Telephone
Laboratories site in Holmdel, New Jersey. Bell Labs wanted to
investigate atmospheric and ionospheric properties using
"short waves" (wavelengths of about 10-20 meters) for use in transatlantic radio telephone service. As a radio
engineer, Jansky was assigned the job of investigating sources of static that might
interfere with radio voice transmissions.
Radio astronomy
Jansky built an antenna designed to receive radio waves at a frequency of 20.5 MHz (wavelength about 14.6 meters). It was mounted on a
turntable that allowed it to rotate in any direction, earning it the name "Jansky's merry-go-round". It had a diameter of
approximately 100 ft. and stood 20 ft. tall. By rotating the antenna on a set of four Ford Model-T tires, the direction of a received signal could be pinpointed. A small shed to the side of the
antenna housed an analog pen-and-paper recording system. (A photograph of Jansky with his
antenna in Holmdel appears in the documentation of a related device, the Reber Radio
Telescope.[1])
After recording signals from all directions for several months, Jansky eventually categorized them into three types of static:
nearby thunderstorms, distant thunderstorms, and a faint steady hiss of unknown origin. He spent over a year investigating the
source of the third type of static. The location of maximum intensity rose and fell once a day, leading Jansky to initially
surmise that he was detecting radiation from the Sun.
After a few months of following the signal, however, the brightest point moved away from the position of the Sun. Jansky also
determined that the signal repeated on a cycle of 23 hours and 56 minutes. This four-minute lag is a typical astronomical
characteristic of any "fixed" object located far from our solar system (see sidereal day).
By comparing his observations with optical astronomical maps, Jansky concluded that the radiation was coming from the
Milky Way and was strongest in the direction of the center the galaxy, in the constellation of Sagittarius.
His discovery was widely publicized, appearing in the New York Times of
May 5 1933. He published his classic paper "Electrical disturbances
apparently of extraterrestrial origin" in Proc. IRE in 1933. This paper was re-printed in Proc. IEEE in 1984 (for their
centennial issue, where they note the research most likely would have won a Nobel prize, had not the author died young) and again
in 1998, for the first centennial of radio. Jansky wanted to follow up on this discovery and investigate the radio waves from the
Milky Way in further detail. He submitted a proposal to Bell Labs to build a 30 meter diameter dish antenna with greater sensitivity that would allow more careful measurements of the structure and
strength of the radio emission. Bell Labs, however, rejected his request for funding on the grounds that the detected emission
would not significantly affect their planned transatlantic communications system. Jansky was re-assigned to another project and
did no further work in the field of astronomy.
Follow-up
Several scientists were interested by Jansky's discovery, but radio astronomy remained a dormant field for several years, due
in part to Jansky's lack of formal training as an astronomer. His discovery had come in the midst of the Great Depression, and observatories were wary of taking on any new and potentially
risky projects.
Two men who learned of Jansky's 1933 discovery were of great influence on the later development of the new study of radio
astronomy: one was Grote Reber, a radio engineer who singlehandedly built a radio telescope in his Illinois back yard in 1937 and did the first systematic survey of astronomical radio waves. The second was Prof. John D. Kraus, who, after World War II, started a radio observatory
at Ohio State University and wrote a textbook on radio astronomy, still considered
a standard by many radio astronomers.
Legacy
In honor of Jansky, the unit used by radio astronomers for the strength (or flux density) of
radio sources is the Jansky (1 Jy = 10-26 W m-2 Hz-1).
Jansky crater on the Moon is also named after him. The
NRAO postdoctoral
fellowship program is named after Karl Jansky.
A full-scale replica of Jansky's original rotating telescope is located on the grounds of the NRAO site in Green Bank, West
Virginia, near a reconstructed version of Grote Reber's 9m dish.
The original site of Jansky's antenna (40.365175 deg. N, 74.163705 deg. W) was determined by Lucent Technologies (the successor of Bell Telephone Laboratories) in the 1990's, and a plaque was place
honoring the achievement. The original site is located at 101 Crawfords Corner Road, Holmdel, New Jersey. As mentioned above, a photograph of Jansky with his antennae appears
in documentation of Reber Radio Telescope.
Jansky noise is named after Jansky, and refers to high frequency
static disturbances of cosmic origin.
Karl Jansky died at age 44 of undisclosed causes in Red Bank, New Jersey.
References
Publications by Karl G. Jansky:
- 1933: "Radio waves from outside the solar system", Nature, 132, p.66.
- 1933: "Electrical phenomena that apparently are of interstellar origin", Popular Astronomy, 41, p.548, Dec.1933.
See also
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