Pluto
Tombaugh's had to systematically get pictures of the night sky in pairs every two weeks.  Then, he had to look at each pair, to see whether any object had shifted.  He used a machine called a blink comparator. He quickly shifted between the different views of each of the plates. This helped him to see whether any object had changed its position. On February 18, 1930, Tombaugh discovered an object which seemed to move from the photographic plates taken on January 23 and January 29 of the year. Another picture taken on January 21 confirmed this. After the observatory did more research about it, news of the discovery was telegraphed to the Harvard College Observatory on March 13, 1930.