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.