Clyde Tombaugh

Clyde Tombaugh was an American astronomer who discovered Pluto in 1930 by comparing photographic plates and later searched for other distant planets.

Lowell Observatory: An astronomical observatory in Flagstaff, Arizona, founded by Percival Lowell in 1894. Tombaugh joined its search for a suspected planet beyond Neptune.
Blink comparator: An optical instrument that rapidly alternates two images of the same sky region to reveal changes between them. Tombaugh used it to make Pluto’s movement stand out across paired plates.
William Pickering: An American astronomer who proposed a trans-Neptunian planet and made predictions that informed searches for it. His proposed planet was among the earlier targets considered at Lowell Observatory.
Pluto: A dwarf planet in the Kuiper belt, discovered in 1930 and reclassified from planet to dwarf planet in 2006. It is the celestial body Tombaugh discovered and the focus of his lasting fame.
Percival Lowell: An American astronomer who founded Lowell Observatory and promoted the search for a planet beyond Neptune. His prediction of a trans-Neptunian planet shaped the search Tombaugh continued.
Astronomical photographic plate: A light-sensitive glass plate used to record images of celestial objects. Plates preserved the star fields that Tombaugh compared for a moving point.
Elizabeth Williams: An American mathematician who worked at Lowell Observatory and calculated positions for the search for Planet X. Her calculations helped establish where observatory searches looked for a distant planet.
Kuiper belt: A region beyond Neptune containing icy bodies, including Pluto, that orbit the Sun. Pluto’s place in this population changed how astronomers understood its classification.
Planet X: A hypothesized planet beyond Neptune proposed to explain apparent irregularities in the orbits of Uranus and Neptune. The search for Lowell’s Planet X gave Tombaugh the task that led to Pluto.
Proper motion: The apparent angular change in a star’s position across the sky over time. The same positional-change principle helped distinguish Pluto from fixed stars.

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