(William) (1906-) US astronomer who discovered the planet Pluto 1930.
Tombaugh, born in Streator, Illinois, became an assistant at the Lowell Observatory in Flagstaff, Arizona, in 1929, and photographed the sky in search of an undiscovered but predicted remote planet.
The new planet would be dim, so each photograph could be expected to show anything between 50,000 and 500,000 stars. And, because of its distance from the Earth, any visible motion would be very slight. Tombaugh solved the problem by comparing two photographs of the same part of the sky taken on different days. The photographic plates were focused at a single point and alternately flashed rapidly on to a screen. A planet moving against the background of stars would appear to move back and forth on the screen. Tombaugh found Pluto on 18 Feb 1930, from plates taken three weeks earlier. He continued his search for new planets across the entire sky; his failure to find any placed strict limits on the possible existence of planets beyond Pluto.