In the summer of 1930, a young radio engineer by the name of Karl Jansky developed a new technology that opened the heavens for observation far beyond what had been previously possible. With this humble beginning, radio astronomy was born.

(Image: Kenny Tong/Shutterstock)

The Birth of Radio Astronomy

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. (Image: Saber1983 at English Wikipedia/Public domain)

In the summer of 1930, the technology of communicating by radio over large distances was just a few years old. A young radio engineer working at Bell Telephone Laboratories named Karl Jansky was given the assignment of finding out what natural radio signals might interfere with transatlantic telephone communications. He had an antenna that could scan the horizon, looking for sources of these interfering signals.

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The Antenna

It was a wild-looking antenna made of brass piping mounted on wheels from a Model T Ford. It rotated around once every 20 minutes scanning the horizon. The antenna and receiver worked at a low frequency by today’s standards, around 20 MHz, but it was state of the art in those days. Jansky’s apparatus recorded signals with a pen and moving chart, and he could also listen in with headphones.

Jansky had everything up and running by 1932, and he saw—and heard—a lot of radio signals from thunderstorms, both near and distant. If you’ve ever tried to listen to the radio during a thunderstorm, you appreciate that lightning makes lots of sharp bursts of radio static, making it tough to communicate through that.

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But Jansky saw something else: a faint but persistent radio hiss that swept across the sky each day. His antenna couldn’t fix the precise location of the extra signal, but before long he realized that every day it was appearing a bit earlier. After a month, it had shifted two hours earlier.

Full-size replica of Jansky’s radio telescope, now at the National Radio Astronomy Observatory. (Image: Astrochemist/Public domain)

Signals From Space

He thought this extra signal might be some odd emanation from the sun, but by chance, there was a partial solar eclipse in New Jersey in August 1932 and the signals did not disappear. After he had an entire year’s data in hand, he finally understood that the signals were coming from a fixed point in space, outside of the solar system. He had discovered radio waves originating from the center of the Milky Way.

Jansky discovered radio waves originating from the center of the Milky Way. (Image: Stefano Garau/Shutterstock)

Jansky presented his work at a couple of meetings in 1933, and the news made the front page of the New York Times.

As monumental as this discovery was, it did not cause astronomers around the world to drop their photographic plates and rush to build radio receivers. Jansky gave some lectures and presentations at conferences; there was the article in the New York Times and reports published in professional journals, but almost nobody rushed to follow it up.

In part, this was because Jansky’s discovery came out of a project in engineering. Dr. Woody Sullivan, a historian of radio astronomy explained it this way: “…this basic discovery was a misfit. Neither fish nor fowl, it was unable to be appreciated by either the scientists or engineers, and therefore lay untouched as an isolated curiosity.”

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Jansky’s Contributions

Of course, radio engineers knew about the discovery, but they viewed it from their perspective, as something to take into account when building receivers. When work on radar began in Great Britain in the 1930’s, engineers knew that besides the return signal reflected off an incoming aircraft, they could also expect to get some signal in their antennas from this celestial static. They called it “Jansky noise”.

Karl Jansky had other work to do at Bell Labs, and he never did much follow-up work on his celestial static. A few scientists at other institutions made attempts to detect the radio emissions, but it was clear that progress would require large antennas, and there wasn’t the enthusiasm or the funding to build them.

Karl Jansky died in 1950 at the age of 44. We can speculate that had he been able to continue his work, or had he lived long enough to witness the explosion of radio astronomy activities that happened in the late 1950s, he would have certainly been awarded the Nobel prize. Surely his discovery was one of the greatest in astronomy over the last 100 years. But he died young.

To honor his achievement, the unit of radio wave intensity from astronomical objects is called the Jansky.

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A wavelength of 14.6 meters is a frequency of about 20 MHz. Our unit of frequency is the hertz, named after the German scientist Heinrich Hertz. A radio signal that has a frequency of one hertz means that one wave passes by each second. In speaking of frequencies, we use prefixes like kilohertz—one thousand times per second; megahertz (MHz)—one million times a second; and gigahertz—one billion times a second. A gigahertz is one thousand megahertz. So, Jansky’s antenna picked waves that were passing by at twenty-million times a second.

Early radio work began in the kilohertz regime. The technology was easier at the lowest frequencies because those waves are long. As radio engineers began pushing their way to higher frequencies, they began talking about “shortwave radio”. It’s all relative, of course. By today’s standards, we certainly don’t think of a wavelength of 14.6 meters as being “short”, but Jansky did. Today, radio astronomers work at frequencies ranging from MHz to THz; a teraHertz is a million megahertz. Wavelengths can be smaller than a millimeter. Still, old names often stick, and the phrase “shortwave radio” is still used.

Radio frequency spectrum showing the different wave lengths used by radio engineers. (Image: Fouad A. Saad/Shutterstock)

Common Questions About Radio Astronomy

This article was updated on June 3, 2020