Bell Labs Engineer Karl Jansky Detects Radio Waves From Milky Way Center in 1932

August 30, 2026 at 06:00 PM2 min

This digest was compiled by AI from multiple sources — links to the originals are below.

Bell Labs engineer Karl Jansky detected radio waves from the center of the Milky Way in 1932 while investigating static disrupting transatlantic phone calls. He traced the signal to the constellation Sagittarius after noticing it peaked on a 23-hour, 56-minute cycle. The discovery marked the birth of radio astronomy.

Key Facts

Karl Jansky built a directional antenna about 100 feet across and 20 feet tall, mounted on four wheels from a Ford Model T, for less than $1,000.
The antenna, nicknamed 'Jansky's merry-go-round,' rotated on a circular track to scan the sky in every direction.
Jansky noticed the hiss peaked on a cycle of 23 hours and 56 minutes, a sidereal day, indicating a source beyond the solar system.
In a February 1933 letter to his father, Jansky wrote that the static came from a direction fixed in space, not from the sun.
By comparing readings with star maps, Jansky traced the strongest signal to the constellation Sagittarius, toward the Milky Way's center.

The Investigation

Bell Labs tasked Karl Jansky with identifying the source of static disrupting its transatlantic radio-telephone service in the early 1930s. Jansky, an engineer trained at the University of Wisconsin who joined Bell Labs in 1928, ruled out thunderstorms because the hiss appeared on clear days. He also ruled out the sun after noticing the timing drifted away from noon by about four minutes each day. The signal did not follow the 24-hour clock, suggesting an origin beyond Earth's daily rotation relative to the sun.

The Antenna

Jansky built a directional antenna about 100 feet across and 20 feet tall at a field site in Holmdel, New Jersey. The antenna was mounted on four wheels salvaged from a Ford Model T, allowing it to rotate on a circular track. The entire rig cost less than $1,000, and coworkers nicknamed it 'Jansky's merry-go-round.' For nearly a year, the antenna recorded a hiss that came and went on a schedule nothing on Earth kept.

The Discovery

Jansky noticed the hiss peaked on a cycle of 23 hours and 56 minutes, a sidereal day, not the 24-hour solar day. A sidereal day is the time Earth takes to rotate once relative to distant stars, indicating a source beyond the solar system. In a February 1933 letter to his father, Jansky wrote that the static came from a direction fixed in space, not from the sun. By comparing readings with star maps, he traced the strongest signal to the constellation Sagittarius, toward the center of the Milky Way.