For millennia, astronomy relied solely on visible light (the small band of the electromagnetic spectrum detectable by the human eye). The universe was thought to be largely silent. This perception was shattered in the early 1930s by the American electrical engineer Karl Jansky (1905–1950). His work, initially focused on finding the source of radio static, inadvertently led to the discovery of cosmic radio waves, marking the birth of a new and profoundly powerful field: Radio Astronomy.

Jansky was employed by Bell Telephone Laboratories to investigate sources of static interference that plagued transatlantic short-wave radio voice transmissions. To precisely locate these noise sources, he built a specialized, rotatable antenna array—nicknamed the "carousel" due to its circular shape and ability to turn 360 degrees.

• Three Sources of Static: Jansky identified three main sources of radio static: nearby thunderstorms, distant thunderstorms, and a faint, persistent hiss that he initially couldn't place.

Jansky spent over a year tracking the direction of this faint, unexplained hiss. He noted that the direction from which this static originated slowly shifted over the course of a day, repeating a cycle every 23 hours and 56 minutes.

• Sidereal Day: This period is not a 24-hour solar day, but the length of a sidereal day—the time it takes for the Earth to complete one rotation relative to the distant stars.

• The Conclusion: Jansky correctly deduced that the source of the radio waves was extraterrestrial (outside the solar system) and fixed in position relative to the Milky Way galaxy.

He identified the peak intensity of the radio signal as coming from the constellation Sagittarius, pointing directly toward the dense, central region of our own Milky Way Galaxy.

Jansky’s discovery, published in 1933, proved that the universe was radiating energy in parts of the electromagnetic spectrum that were previously unknown to astronomers. It opened the universe up to be studied via its radio emissions.

• Initial Reception: Despite the revolutionary nature of the discovery, Bell Labs discouraged Jansky from pursuing astronomy, preferring him to focus on telecommunications engineering. The astronomical community was slow to capitalize on the finding, as they lacked the necessary equipment and experience with radio waves.

The potential of radio astronomy was later championed by American amateur astronomer Grote Reber. Frustrated by the lack of institutional interest, Reber built the first dedicated parabolic radio telescope (a 31-foot dish) in his backyard in Illinois in 1937.

• Mapping the Sky: Reber spent years systematically mapping the sky's radio emissions, independently confirming Jansky’s findings and creating the first detailed radio maps of the Milky Way, identifying not only the galactic center but other strong sources, establishing the technical foundations of the field.

The work of Jansky and Reber proved that objects that appear dim in visible light might be extremely bright in radio waves. Radio astronomy later led to the critical discoveries of quasars (highly active galactic nuclei), pulsars (rapidly rotating neutron stars), and the Cosmic Microwave Background (Article 126)—the echo of the Big Bang.

In his honor, the fundamental unit of flux density (power received per unit area per unit frequency) in radio astronomy is the jansky ($\text{Jy}$).

In Conclusion: Karl Jansky's accidental discovery of persistent cosmic radio waves emanating from the center of the Milky Way, confirmed by tracking the source to the sidereal day, irrevocably broadened the spectrum of astronomical observation. His work, despite initial neglect, launched the entirely new discipline of Radio Astronomy, which has been essential for charting the structure of the cosmos and discovering its most violent and distant objects.

For millennia, astronomy relied solely on visible light (the small band of the electromagnetic spectrum detectable by the human eye). The universe was thought to be largely silent. This perception was shattered in the early 1930s by the American electrical engineer Karl Jansky (1905–1950). His work, initially focused on finding the source of radio static, inadvertently led to the discovery of cosmic radio waves, marking the birth of a new and profoundly powerful field: Radio Astronomy.

Jansky was employed by Bell Telephone Laboratories to investigate sources of static interference that plagued transatlantic short-wave radio voice transmissions. To precisely locate these noise sources, he built a specialized, rotatable antenna array—nicknamed the "carousel" due to its circular shape and ability to turn 360 degrees.

• Three Sources of Static: Jansky identified three main sources of radio static: nearby thunderstorms, distant thunderstorms, and a faint, persistent hiss that he initially couldn't place.

Jansky spent over a year tracking the direction of this faint, unexplained hiss. He noted that the direction from which this static originated slowly shifted over the course of a day, repeating a cycle every 23 hours and 56 minutes.

• Sidereal Day: This period is not a 24-hour solar day, but the length of a sidereal day—the time it takes for the Earth to complete one rotation relative to the distant stars.

• The Conclusion: Jansky correctly deduced that the source of the radio waves was extraterrestrial (outside the solar system) and fixed in position relative to the Milky Way galaxy.

He identified the peak intensity of the radio signal as coming from the constellation Sagittarius, pointing directly toward the dense, central region of our own Milky Way Galaxy.

Jansky’s discovery, published in 1933, proved that the universe was radiating energy in parts of the electromagnetic spectrum that were previously unknown to astronomers. It opened the universe up to be studied via its radio emissions.

• Initial Reception: Despite the revolutionary nature of the discovery, Bell Labs discouraged Jansky from pursuing astronomy, preferring him to focus on telecommunications engineering. The astronomical community was slow to capitalize on the finding, as they lacked the necessary equipment and experience with radio waves.

The potential of radio astronomy was later championed by American amateur astronomer Grote Reber. Frustrated by the lack of institutional interest, Reber built the first dedicated parabolic radio telescope (a 31-foot dish) in his backyard in Illinois in 1937.

• Mapping the Sky: Reber spent years systematically mapping the sky's radio emissions, independently confirming Jansky’s findings and creating the first detailed radio maps of the Milky Way, identifying not only the galactic center but other strong sources, establishing the technical foundations of the field.

The work of Jansky and Reber proved that objects that appear dim in visible light might be extremely bright in radio waves. Radio astronomy later led to the critical discoveries of quasars (highly active galactic nuclei), pulsars (rapidly rotating neutron stars), and the Cosmic Microwave Background (Article 126)—the echo of the Big Bang.

In his honor, the fundamental unit of flux density (power received per unit area per unit frequency) in radio astronomy is the jansky ($\text{Jy}$).

In Conclusion: Karl Jansky's accidental discovery of persistent cosmic radio waves emanating from the center of the Milky Way, confirmed by tracking the source to the sidereal day, irrevocably broadened the spectrum of astronomical observation. His work, despite initial neglect, launched the entirely new discipline of Radio Astronomy, which has been essential for charting the structure of the cosmos and discovering its most violent and distant objects.