Working at Lowell Observatory in Flagstaff, Arizona, self-taught astronomer Clyde Tombaugh spent months blink-comparing pairs of photographic plates searching for the hypothesized 'Planet X.' On plates from January 23 and 29, 1930, he spotted a faint point of light that shifted position — a new planet. The observatory announced the find on March 13, the anniversary of both Percival Lowell's birthday and Uranus's discovery, and the world celebrated a ninth planet, later reclassified as a dwarf planet in 2006.
Clyde Tombaugh had no formal astronomy training when Lowell Observatory hired him in 1929 specifically to hunt for a hypothetical trans-Neptunian planet, whose existence founder Percival Lowell had predicted based on supposed irregularities in Uranus's orbit. The job was numbingly tedious: photograph the same patch of sky nights apart, then flick back and forth between the plates on a machine called a blink comparator, looking for any point of light that moved against the fixed stars.
On February 18, 1930, after nearly a year of this work, Tombaugh found it — a faint dot that had shifted position between plates taken on January 23 and January 29. Observatory staff spent weeks confirming the object before announcing the discovery on March 13, deliberately chosen because it fell on Percival Lowell's birthday and the anniversary of Herschel's discovery of Uranus 149 years earlier.
An 11-year-old English girl named Venetia Burney suggested the name Pluto, after the Roman god of the underworld, reasoning that a cold, dark, distant world deserved a name from the underworld; her grandfather passed the suggestion to the Lowell staff via the Royal Astronomical Society. Pluto held planetary status for 76 years until the International Astronomical Union reclassified it as a dwarf planet in 2006, a decision Tombaugh, who died in 1997, never lived to contest.