How did Karl Jansky discover radio signals from the centre of the Milky Way?

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Telescope Aerial view of the Keck Observatory's twin domes, which are opened to reveal the telescopes. Keck II is on the left and Keck I on the right.

In 1928, American engineer Karl Jansky began working at Bell Telephone Laboratories with the task of identifying sources of interference affecting telephone communications. To investigate these signals, he constructed a linear, directional antenna.

While he successfully identified various forms of interference, one persistent, unidentified signal remained. After months of study, Jansky discovered in 1931 that this interference originated from an extraterrestrial source. By the spring of 1932, he determined that the radio waves were emanating from the direction of the constellation Sagittarius. Because previous research by Harlow Shapley and Jan Oort had established this region as the center of the Milky Way Galaxy, Jansky concluded that the signals were coming from the galactic core. This discovery marked the birth of radio astronomy.1