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Radio astronomy began with an engineer trying to fix telephone calls. In the early 1930s, a young physicist at Bell Telephone Laboratories was asked to find out what caused the static on transatlantic radio links. Most of it came from thunderstorms. One part did not, and following it led him out of the solar system. This post tells Karl Jansky's story, explains the clue that gave the source away, works through the numbers, and lists what a student can learn from it.
Who was Karl Jansky?
The National Radio Astronomy Observatory's history page on Jansky gives the essentials. Karl Guthe Jansky was born in Norman, Oklahoma, on 22 October 1905 and died on 14 February 1950. He graduated in physics from the University of Wisconsin and joined Bell Telephone Laboratories in Holmdel, New Jersey, in 1928.
Bell Labs wanted to use short waves, about 10 to 20 metres long, for radio telephone service across the Atlantic. Jansky's job was to identify sources of static that might interfere.
How did Jansky find radio waves from space?
He built an antenna designed to receive radio waves at 20.5 MHz and mounted it on a turntable so it could be pointed in any direction. It became known as "Jansky's merry-go-round". After months of recording, NRAO says, he sorted the static into three kinds: nearby thunderstorms, distant thunderstorms, and "a faint steady hiss of unknown origin".
His first results appeared in Directional studies of atmospherics at high frequencies (1932). The third kind took over a year to explain. It rose and fell once a day, so at first he suspected the Sun. But over a few months the peak drifted away from the Sun's position. The signal repeated "not every 24 hours, but every 23 hours and 56 minutes". He concluded that it came from the Milky Way and was strongest towards the galaxy's centre, in the constellation Sagittarius.
He published the result in 1933 as Electrical disturbances apparently of extraterrestrial origin in the Proceedings of the Institute of Radio Engineers, with a short note in Nature, Radio waves from outside the solar system.
Why does a 23-hour 56-minute repeat point to the stars?
Earth spins on its axis and also moves around the Sun, about 360° in a year, or roughly 1° a day. To bring the Sun back to the same place in the sky, Earth must turn about 1° more than one full spin. A full spin, measured against the distant stars, is the sidereal day, about 23 hours 56 minutes. The solar day, measured against the Sun, is 24 hours.
So a source among the stars rises about 4 minutes earlier each solar day. After a month that adds up to about 2 hours, and after six months about 12 hours, which is why Jansky's peak drifted steadily away from the Sun.
Worked example: the numbers behind Jansky's discovery
1. Wavelength of his receiver. λ = c / f = (3.00 × 10^8 m/s) ÷ (20.5 × 10^6 Hz) = 14.6 m. That matches NRAO's "about 14.5 meters" within rounding, and sits in the short-wave band Bell Labs cared about.
2. Drift over a year. 24 h − 23 h 56 min = 4 min per day. Over 365 days: 365 × 4 = 1,460 minutes, or about 24.3 hours. The star clock gains roughly one whole day per year, as it must, since Earth makes one extra turn relative to the stars for each trip round the Sun. (The true difference is slightly under 4 minutes, which brings the total closer to exactly one day.)
3. How weak is a radio source? NRAO gives the unit named after him: one jansky is a flux density of 10^-26 W per square metre per hertz. Sample numbers: a source of 100 Jy, a dish collecting area of 100 m² and a receiver bandwidth of 1 MHz.
Power = 100 × 10^-26 × 100 × 10^6 = 10^-16 W.
That is a tenth of a femtowatt, which is why radio telescopes need large dishes and sensitive receivers.
What mistakes do students make with these numbers?
Dropping the "mega". 20.5 MHz is 20.5 × 10^6 Hz. Forgetting the 10^6 gives a wavelength in kilometres.
Treating the jansky as a power. It is power per area per unit bandwidth. Multiply by both area and bandwidth to get watts.
Mixing up sidereal and solar days. The sidereal day is shorter. Stars rise earlier each night, not later.
Radio waves and their uses sit in the electromagnetic spectrum unit of the JEE (Main) 2026 syllabus, and λ = c/f conversions appear often. Our guide to the electromagnetic spectrum covers the bands. The NCERT chapter on electromagnetic waves credits Hertz with first demonstrating these waves in the laboratory; our post on Heinrich Hertz and radio waves tells that story.
What happened next, and what can a student take from it?
NRAO records that Jansky proposed a 100-foot dish to study the signal further, but Bell Labs had its answer, since the hiss was not a problem for telephone links, and moved him to other work. Grote Reber, who built a radio telescope in his back yard in 1937, and John Kraus carried the field forward.
Do the boring job well. A careful survey of static, done for an engineering reason, produced a discovery.
Watch for the small mismatch. Four minutes a day separated "the Sun" from "the galaxy".
Follow the clue across months. The drift only became clear with long records.
Talk across fields. An engineer's data changed astronomy because others took it up.
A practice routine: convert five broadcast frequencies (FM, AM, Wi-Fi, mobile and Jansky's 20.5 MHz) to wavelengths, then work out how many hours earlier a star rises after 45 days. If wavelength, frequency and the spectrum are still blurry, the Fermi Concept Library covers every concept in physics, so you can revise electromagnetic waves before the next problem set.
Next step: note the time a bright star rises tonight and check it a week later; it should be about half an hour earlier.
