January 17: Clyde Tombaugh

Clyde William Tombaugh 

February 4, 1906 – January 17, 1997

Clyde Tombaugh was an American astronomer. Tombaugh is best known for discovering the dwarf planet Pluto in 1930, but also discovered many asteroids.

While a young researcher working for the Lowell Observatory in Flagstaff, Arizona, Tombaugh was given the job to perform a systematic search for a trans-Neptunian planet (also called Planet X), which had been predicted by Percival Lowell and William Pickering.

Tombaugh used the observatory's 13-inch astrograph to take photographs of the same section of sky several nights apart. He then used a blink comparator to compare the different images. When he shifted between the two images, a moving object, such as a planet, would appear to jump from one position to another, while the more distant objects such as stars would appear stationary. Tombaugh noticed such a moving object in his search, and subsequent observations showed it to be the object we call Pluto. The discovery was made on Tuesday, February 18, 1930, using images taken in January of the same year.

Tombaugh discovered a total of 14 asteroids, during his search for Pluto and years of follow-up searches looking for another candidate for the postulated Planet X. Tombaugh is also credited with the discovery of comet C/1931 AN, though its orbit is currently unknown. The asteroid 1604 Tombaugh, discovered in 1931, is named after him.