In 1931, what did Karl Jansky discover?

• A. Radio emissions from the sky
• B. X-rays in space
• C. Gamma rays from the Sun
• D. Neutrinos from the cosmos

Answer: (A)

This is about the first detection of radio waves coming from space, the moment radio astronomy began. Karl Jansky built a large, very sensitive antenna at Bell Telephone Laboratories to study radio noise and figure out which signals were from Earth and which came from the sky. After careful observations, he found a strong, steady radio signal that moved with the rotation of the Earth and pointed toward the center of the Milky Way. Because the signal’s direction in the sky mattered, and it wasn’t just a daily Earth-based glitch, he concluded it originated in space. This marked the first time humans detected radio emissions from celestial sources, launching a whole new way of studying the universe.

Other options point to types of observations that weren’t what Jansky detected. X-rays from space and gamma rays from the Sun, as well as neutrinos from cosmic sources, required different instruments and came later in the history of astronomy.