Discovery of Pluto

Discovery of Pluto

. A region of the constellation Gemini, photographed by Clyde W. Tombaugh on 23 January 1930 (left), and the same region photographed six days later (right). When comparing the two plates on 18 February 1930 with a blink microscope, Tombaugh noticed an object (arrows) on the second plate that had changed its location with respect to the background stars since the first plate was taken. This was a previously unknown object that had to belong to the solar system. Because of its slow apparent motion across the sky, the planet images were separated by just 3.5 millimeters on the two photographs. (Courtesy of the Lowell Observatory.).

Copyright 2010, Professor Kenneth R. Lang, Tufts University