Pluto

Astronomers suspected the existence of a distant planet beyond Neptune due to observed discrepancies in Uranus's orbit. Pluto was eventually discovered by taking two pictures of the same small part of the sky on two different days, and looking for differences.
 Clyde William Tombaugh United States 1930 Pluto

Astronomers suspected the existence of a distant planet beyond Neptune due to observed discrepancies in Uranus's orbit. Pluto was eventually discovered by taking two pictures of the same small part of the sky on two different days, and looking for differences.