Lowell Observatory, located just above Flagstaff, Arizona, is one of the most important astronomical observatories in the world. Here, Edwin Hubble found evidence that the universe is expanding, and a young inexperienced astronomer by the name of Clyde Tombaugh discovered Pluto in 1930.
This is the telescope that Tombaugh used to photograph and discover Pluto. An observatory volunteer demonstrates the operation of the photography plate on the telescope.
The discovery of Pluto was based on meticulous observations of the same area of the sky over the months, as well as calculations of expected and unexpected gravitational forces. Neptune's discovery had been based on gravitational force calculations as well, though unlike Neptune, Pluto is too small to effect gravitational forces on other planets. In fact Pluto is so small, and so different from the outer planets (and so similar to other small objects beyond Neptune that are now known), that it was demoted to a "dwarf planet" in 2006.