The Discovery of Pluto

On February 18, 1930, Clyde Tombaugh discovered Pluto while working at Lowell Observatory. He made the discovery by carefully comparing two photographic plates taken six days apart and noticed a tiny point of light that had shifted position against the background of fixed stars.

The search that led to this breakthrough had actually begun decades earlier. About thirty years before Tombaugh’s discovery, Percival Lowell initiated a hunt for a possible ninth planet at the observatory in Flagstaff. Astronomers believed that slight irregularities in the orbits of Uranus and Neptune were caused by the gravitational pull of an unknown planet. Finding it required examining countless star images in search of a faint object that moved over time. Lowell himself never succeeded.

Because Tombaugh was from Kansas, his discovery made Pluto the only planet identified by an American.

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