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What tool did Clyde Tombaugh use to compare photographs of the sky?
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Pluto
Pluto as observed by the New Horizons spacecraft, July 13, 2015.
To discover Pluto, Clyde Tombaugh used an instrument called a blink comparator to examine photographic plates of the sky.1
A blink comparator is an optical device that allows two images of the same area of the sky, taken at different times, to be viewed alternately in rapid succession within a single visual field. By "blinking" between the two plates, any celestial body that had changed position—such as a planet—would appear to shift against the stationary background of stars, making it easier to identify.21