The planet Pluto turns 75 this month. Clyde Tombaugh discovered the ninth planet in the solar system on the afternoon of February 18, 1930 while he meticulously examined a pair of deep sky photographs at Lowell Observatory. Tombaugh exposed the photographs on two nights in late January using the Observatory's 13-inch Abbott Lawrence Lowell Telescope. Then, as part of the carefully planned and executed planet search, Tombaugh "blinked" the two exposures using a machine called a comparator, looking for motion of objects captured on film.