In the early 1930s, Karl Jansky was not searching for signals from distant stars. He was working on a practical communications problem for Bell Telephone Laboratories: identifying the sources of static that could interfere with transatlantic radiotelephone service.
His investigation required him to determine where different radio disturbances came from and whether they could compromise reliable communication.
Most of the noise had an earthly explanation. Thunderstorms produced recognizable interference, whether they occurred nearby or far beyond the horizon. But Jansky also detected a faint, persistent hiss that did not behave like the other signals.
By carefully tracking its direction and timing, he discovered that the mysterious noise was coming from beyond the solar system and was strongest toward the center of the Milky Way. What began as an investigation of radio interference had opened an entirely new way to observe the universe.
An Engineer Surrounded by Radio
Karl Guthe Jansky was born on October 22, 1905, in what was then the Oklahoma Territory. Engineering and scientific work were already part of his family. His father, Cyril Jansky, was an electrical engineering professor at the University of Wisconsin, while his older brother, Cyril Jr., became a radio engineer who helped develop some of the earliest radio transmitters used by universities in the United States.
Jansky studied physics at the University of Wisconsin and graduated in 1927. After an additional year of graduate study, he joined Bell Laboratories in 1928. He was assigned to its facility in Holmdel, New Jersey, where engineers were studying the possibilities and limitations of shortwave radio for long-distance telephone communication.
Shortwave signals could travel across enormous distances, making them attractive for transatlantic service. However, their reliability depended on understanding the many natural and human-made sources of radio noise that could obscure speech. Bell Labs needed to know when interference occurred, where it originated, and how seriously it might affect a communications link.
Jansky’s role was to find out.
Building the “Merry-Go-Round”
To investigate the problem, Jansky constructed a directional antenna designed to receive radio waves at approximately 20.5 MHz, corresponding to a wavelength of about 14.6 meters. The structure was roughly 100 feet wide and 20 feet tall.
The antenna was mounted on a rotating platform supported by wheels from a Ford Model T. By turning the entire structure, Jansky could determine the direction from which a signal was arriving. Its appearance and circular movement earned it the nickname “Jansky’s merry-go-round.”
The antenna was unusual, but its purpose was straightforward. Jansky was not simply measuring how much noise appeared at the receiver. He needed to connect that noise with its source. Directional measurements allowed him to observe how the strength of a signal changed as the antenna rotated.
For months, Jansky recorded signals arriving from different directions. The results appeared as lines traced across long paper charts. He studied those records, compared the timing of repeated disturbances, and gradually separated the noise into recognizable categories.
Following the Stars Instead of the Sun
A solar day lasts 24 hours, based on Earth’s rotation relative to the Sun. But Earth also moves along its orbit while it rotates. Relative to distant stars, one complete rotation takes approximately 23 hours and 56 minutes. This interval is known as a sidereal day.
The unexplained signal followed the sidereal cycle.
Over time, its strongest position moved away from the apparent position of the Sun. Jansky concluded that the source could not be solar. It had to be associated with something much farther away, beyond the solar system.
By comparing his directional measurements with astronomical observations, he determined that the signal was coming from the Milky Way and was strongest in the direction of the constellation Sagittarius, toward the center of the galaxy.
In 1933, Jansky presented his findings in a paper describing radio disturbances of extraterrestrial origin. His discovery received widespread public attention, including coverage in The New York Times. For the first time, researchers had demonstrated that radio waves arriving from outside the solar system could be detected from Earth.
Astronomy had traditionally depended on visible light. Jansky showed that the universe could also be studied through radio emissions that human eyes could never see.
A Discovery Without an Immediate Successor
Jansky understood that his observations had opened a promising field of research. He reportedly proposed constructing a much larger dish antenna—approximately 100 feet in diameter—to study the signals in greater detail.
Bell Labs, however, had approached the project as a telecommunications investigation. Jansky had determined that the extraterrestrial signal did not present a major obstacle to transatlantic radio service. From the company’s perspective, the original engineering question had been answered. He was reassigned to other communications work and did not continue his astronomical observations.
Professional astronomers also showed limited immediate interest. Radio engineering was unfamiliar to many observatories, suitable equipment was expensive, and the Great Depression made funding new experimental programs particularly difficult.
A few researchers recognized the importance of Jansky’s results. Most notably, American radio engineer Grote Reber constructed a radio telescope in his backyard several years later. Reber confirmed and expanded upon Jansky’s observations, producing an early map of radio emissions across the sky.
The scientific field Jansky had unintentionally initiated would eventually become known as radio astronomy.
Work Beyond the Milky Way Discovery
Although Jansky did not return to sustained astronomical research, his later career continued to involve advanced radio and communications technology.
During the Second World War, he contributed to defense-related projects, including work involving the electronic detection of submarines. He later participated in the development of microwave relay systems for long-distance telephone communication. According to the National Radio Astronomy Observatory, he also became one of the early engineers to use the newly developed transistor in low-noise amplifiers.
Chronic health problems affected Jansky throughout his life. He died in 1950 at only 44 years old, before radio astronomy achieved the scientific recognition it would receive in the following decades.
A Name Written Into Radio Astronomy
Jansky’s contribution became increasingly important as researchers developed more sensitive receivers and larger radio telescopes. Radio observations revealed objects and phenomena that were difficult or impossible to study using visible light alone, from the structure of our galaxy to pulsars, quasars, and the faint radiation left by the early universe.
In 1973, the astronomical community adopted the jansky, abbreviated Jy, as a unit of radio flux density. One jansky represents 10−2610^{-26} watts per square meter per hertz. The unit allows astronomers to describe the strength of radio emissions reaching Earth from celestial sources.
His name also appears on the Karl G. Jansky Very Large Array in New Mexico, one of the world’s leading radio astronomy observatories.
These honors recognize more than a fortunate observation. Jansky designed the equipment, conducted sustained measurements, distinguished among multiple sources of noise, noticed a four-minute difference in the signal’s daily cycle, and followed the evidence until he could explain it.
Listening Closely to the Unexpected
Karl Jansky’s achievement grew out of careful engineering rather than a planned search for a new branch of astronomy. He was asked to characterize interference in a communications system, and he approached the assignment methodically: measure the noise, identify its direction, study its timing, and determine its source.
That process remains relevant wherever sensitive electronics must operate in the presence of competing electromagnetic energy. Engineers must understand which signals belong inside a system, which originate outside it, how energy reaches a receiver, and whether an unexplained response represents a fault, an environmental influence, or something worth investigating.
For those working to control EMI/RFI across electronic systems and assemblies, Jansky’s story offers a broader reminder of why understanding interference matters.
In Jansky’s case, the unexplained radio noise was not merely a disturbance to be cataloged. Hidden within it was evidence that the Milky Way could be heard—and the beginning of an entirely new way to study the universe.
