Karl Jansky and the Merry-Go-Round That Heard the Milky Way

In the early 1930s, in a field near Holmdel, New Jersey, a young Bell Telephone Laboratories engineer built a strange machine. It was a long, gangly frame of antenna elements, about 100 feet long and 20 feet tall, sitting on four wheels salvaged from Ford Model T cars. Every 20 minutes a motor pushed it through one slow, complete turn, like a fairground ride with no riders. His colleagues called it the merry-go-round.

The engineer was Karl Jansky, and his assignment was ordinary: find out what was causing the crackle on the company’s new transatlantic radiotelephone service. He found the thunderstorms he expected. He also found something nobody expected, a faint, steady hiss that seemed to drift around the sky on a schedule of its own.

By the spring of 1933 Jansky had traced that hiss to the center of the Milky Way. It was the first time anyone had picked up radio waves from beyond the solar system, and it opened an entirely new window on the universe. Nearly every radio telescope that came afterward, from a homemade dish in an Illinois backyard to the array that photographed the black hole at the heart of our galaxy, traces its family tree back to that rotating contraption in New Jersey.

A phone call across the ocean

By the late 1920s, a telephone call between New York and London could cross the ocean by shortwave radio. The service was a marvel, but it was expensive, and it was noisy. Static could swamp a conversation without warning, and AT&T, Bell Labs’ parent company, wanted to know where the noise came from and how bad it could get before it planned any expansion.

Jansky was a good fit for the problem. He was born on October 22, 1905, in Norman, in what was then Oklahoma Territory, where his father, Cyril M. Jansky, taught engineering. The family later moved to Madison, where his father became a professor of electrical engineering at the University of Wisconsin. Karl earned his physics degree there in 1927, played on the university ice hockey team, and was elected to Phi Beta Kappa.

He joined Bell Labs in July 1928, at age 22. He had a chronic kidney condition dating from his college years, and the company sent him to its quieter field station at Holmdel rather than its busy New York headquarters. There he was given a deceptively simple job: listen to static on wavelengths of 10 to 20 meters, the band the transatlantic service used, and sort out where it came from.

A merry-go-round on Model T wheels

To do that, Jansky needed an antenna that could tell directions apart. He designed one tuned to a frequency of 20.5 megahertz, a wavelength of about 14.6 meters. It was a type of directional array made from a long row of rectangular frames, and it was most sensitive to signals arriving from one side. If he could swing it around the horizon, he could see which directions were loudest at any given moment.

So he put it on a turntable. The whole structure rode on a circular track on those four Model T wheels, and a small motor turned it through a full circle every 20 minutes. A wire ran to a shed nearby, where a pen recorder inked the strength of the incoming signal onto a moving strip of paper. NRAO later estimated the antenna cost less than $1,000 to build. It looked, as one NRAO history puts it, more like a wooden merry-go-round than like any modern antenna, much less a radio telescope.

For months Jansky let it turn, day and night, and studied the paper. Slowly the static separated into three distinct types. There was the loud crash of nearby thunderstorms, and the weaker, steadier rumble of distant ones. And underneath both was what he described as "a steady hiss type static of unknown origin."

Four minutes early, every day

The hiss was faint, but it was always there, and it was not fixed in any one direction. It rose and fell once a day as the antenna’s favored direction swept past a particular part of the sky. Jansky’s first guess was the Sun. At the time the loudest direction seemed to line up roughly with where the Sun was, and the Sun was the obvious candidate for a natural radio source.

But as the months passed, the two came apart. The peak of the hiss arrived about four minutes earlier every day. Over a year, those minutes add up to a full day, which meant the source was keeping time not with the Sun but with the stars.

Astronomers call this a sidereal day. Because Earth travels around the Sun as it spins, it takes about 23 hours and 56 minutes for a given star to return to the same spot in the sky, four minutes less than an ordinary solar day. A friend and Bell Labs colleague, Albert Melvin Skellett, who had studied astronomy, helped Jansky see what the drift meant. Anything that kept sidereal time had to be fixed among the stars, far outside the solar system.

Jansky compared his directions with star maps. The hiss was strongest when the antenna pointed toward the constellation Sagittarius, the part of the sky where the Milky Way is at its thickest and brightest. The strongest signal of all came at 7:10 in the evening on September 16, 1932. He wrote to his father that the noise was not only extraterrestrial but came from outside the solar system.

That patch of Sagittarius is the same thick band of light that cultures around the world turned into rivers, roads and spilled milk, as we explored in our look at how ancient cultures saw the Milky Way. Jansky was the first person to show that the galaxy could be heard as well as seen.

Front-page news, and then silence

On April 27, 1933, Jansky presented his findings at a meeting of the International Union of Radio Science in Washington, D.C. Within days the story escaped the scientific world. On May 5, 1933, The New York Times ran it on the front page under the headline "New Radio Waves Traced to Centre of the Milky Way." The paper was careful to say there was no evidence the signals were messages from anyone. NBC even broadcast the hiss to its listeners in a program billed as radio sounds from among the stars.

Jansky published his work in the Proceedings of the Institute of Radio Engineers in 1933, under the very cautious title "Electrical disturbances apparently of extraterrestrial origin." In 1935 he suggested that the radio waves came from interstellar gas, produced by what he called the "thermal agitation of charged particles." That was not quite the full answer, but it pointed in the right direction.

The professional astronomers of the day mostly shrugged. Astronomy in the 1930s meant optical telescopes, photographic plates and spectra, the world of Henrietta Leavitt measuring star brightness and Milton Humason measuring galaxy speeds. Few astronomers knew anything about radio receivers, and a faint hiss with no picture attached was hard to fit into their science.

The dish Bell Labs never built

Jansky wanted to go further. He proposed building a dish antenna about 100 feet across, which would have let him map the radio sky in far more detail. Bell Labs said no. The Great Depression was squeezing every budget, and from the company’s point of view the job was done. The cosmic hiss was real, but it was far too weak to bother transatlantic phone calls. There was no business reason to chase it.

Jansky was moved to other projects. He finished a master’s degree at the University of Wisconsin in 1936 with a thesis titled "Star Noise," and he spent World War II working on equipment for detecting submarines, work that earned him an Army-Navy citation. After the war he worked on microwave relay systems and early transistor amplifiers.

His health never fully recovered. He was nominated for the Nobel Prize in Physics in 1948, at a time when few people grasped how important radio astronomy would become. On February 14, 1950, he died in Red Bank, New Jersey, at just 44. As antenna engineer William A. Imbriale later put it: "Had Jansky not died at a very early age, he would undoubtedly have been awarded the Nobel Prize."

A backyard in Wheaton

One person took Jansky’s discovery very seriously. Grote Reber was a young radio engineer and ham operator in Wheaton, Illinois, born in 1911. He had already made radio contact with stations all over the world and felt, by his own account, that there were no more worlds to conquer. Jansky’s hiss gave him a new one.

In 1937, in the yard of his mother’s house, Reber built the dish Bell Labs had turned down, only smaller. It was a parabolic reflector 31.4 feet across, made of 72 wooden rafters covered with 45 sheets of galvanized iron. He spent around $4,000 of his own money and scavenged parts from Model T Fords, just as Jansky had.

His first receivers, tuned to much higher frequencies, heard nothing. Reber kept lowering the frequency, and by 1939 a receiver at 160 megahertz had picked up the cosmic static from the Milky Way, confirming Jansky’s discovery. In the early 1940s Reber produced the first map of the radio sky. It showed the Milky Way glowing in radio waves, with bright spots in the constellations Cygnus and Cassiopeia that later turned out to be a distant radio galaxy and the remains of an exploded star.

For nearly a decade, Reber was essentially the only radio astronomer in the world. His dish later went to the National Bureau of Standards and was eventually rebuilt at the National Radio Astronomy Observatory in Green Bank, West Virginia, where it still stands beside a full-size replica of Jansky’s merry-go-round.

From a hiss to a black hole

World War II changed everything. Radar engineers in Britain and elsewhere learned to build sensitive receivers and big antennas, and some of them noticed the sky was noisy too. In 1942 British army scientist J. S. Hey found that the Sun produced strong radio waves. After the war, those engineers turned their equipment on the heavens, and radio astronomy grew from a curiosity into a major science.

The source Jansky had found was given a name: Sagittarius A, the brightest radio source in that constellation. Then, in February 1974, astronomers Bruce Balick and Robert Brown used a radio interferometer at Green Bank to find something much smaller and sharper inside it, a compact point of radio emission at the very center of the galaxy. It was later named Sagittarius A*, the asterisk added because, as Brown explained, the source was exciting, in the way an excited atom is marked in physics.

Over the following decades, astronomers tracked stars whipping around that point at enormous speeds. Their orbits showed that Sagittarius A* holds about four million times the mass of the Sun in a space smaller than our solar system. Only a black hole fits. In 2020 Reinhard Genzel and Andrea Ghez shared the Nobel Prize in Physics for that work.

On May 12, 2022, the Event Horizon Telescope collaboration released the first image of Sagittarius A*. It was made by linking radio dishes around the world into one Earth-sized telescope, using observations from April 2017. The picture shows a ring of glowing gas around a dark center, the shadow of a black hole about 27,000 light-years away. Seen from Earth, it is roughly the size a doughnut on the Moon would appear.

That glowing ring sits in the same direction Jansky’s merry-go-round pointed at 7:10 p.m. on September 16, 1932. He could not have known what was there. But the hiss on his strip chart was the first sign that something extraordinary was waiting at the center of the galaxy.

Jansky was the first person to show that the galaxy could be heard as well as seen.

The name on every radio dial

Jansky’s legacy is now built into the language of astronomy. In 1973 the International Astronomical Union adopted the jansky, abbreviated Jy, as the unit of radio flux density. One jansky is a tiny amount of energy, 10 to the minus 26 watts per square meter per hertz, which says a lot about how faint the signals radio astronomers chase really are. Measurements of the famous Wow! signal, picked up by Ohio State’s Big Ear telescope in 1977 from roughly the same part of the sky, are given in janskys.

A crater on the Moon and an asteroid carry his name. In 2012 the Very Large Array in New Mexico, 27 radio dishes spread across the Plains of San Agustin, was renamed the Karl G. Jansky Very Large Array.

At Bell Labs, the radio pioneers kept coming. On Crawford Hill, also in Holmdel, a large horn antenna built for satellite communication picked up its own mysterious hiss in 1964 and 1965. Arno Penzias and Robert Wilson eventually traced it to the cosmic microwave background, the afterglow of the Big Bang, and won the Nobel Prize in Physics in 1978.

In 1998 Tony Tyson and Robert Wilson of Lucent Technologies, Bell Labs’ successor, pinned down the exact spot where Jansky’s antenna had stood, and a monument and plaque were placed there. The monument is a stylized sculpture of the merry-go-round, aimed exactly as the real antenna was at 7:10 p.m. on September 16, 1932, the moment it pointed most squarely at the heart of the galaxy.

Listening for yourself

Jansky’s discovery came from patience more than equipment. He did not have a mirror, a lens or a dish. He had a home-built antenna, a pen recorder and the willingness to keep watching the same faint wiggle on paper until it made sense.

Today a hobbyist can do a version of his experiment with a low-cost software-defined radio, a laptop and a homemade antenna. Amateur radio astronomers regularly detect the hydrogen line from the Milky Way at 1,420 megahertz, a signal first detected in 1951 and later used to map the spiral arms of our galaxy. It is the same lesson Jansky left behind. The sky is full of signals, and some of the most important ones sound, at first, like nothing more than noise.

For more places where science and history meet, spin our interactive History Globe, which maps the events we have covered across time.

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