Clyde Tombaugh discovered Pluto on February 18, 1930

Pluto was discovered on February 18, 1930, by Clyde Tombaugh, a 24-year-old assistant at Lowell Observatory in Flagstaff, Arizona, who saw a faint dot shift position between two photographic plates taken six days apart.

Tombaugh was a Kansas farm boy who built his own telescopes. After he mailed some of his own planet drawings to the observatory, director V. M. Slipher offered him a temporary job. He arrived on January 15, 1929, and shoveled coal and gave tours before joining the hunt for the long-sought Planet X.

The work was a grind. He photographed the same patches of sky on different nights through a 13-inch telescope in an unheated dome, then compared pairs of plates on a blink comparator, which flips between two images so that anything that has moved appears to jump. On January 21, 23 and 29, 1930, he photographed the sky around the star Delta Geminorum. About three weeks later, at around 4 p.m. on February 18, he caught the motion on the January 23 and 29 plates.

“Planet ‘X’ (Pluto) at last found!!!” (Clyde Tombaugh, written on the envelope that held the discovery plates)

Pluto turned up close to where Percival Lowell had predicted a ninth planet, but that was luck, not prediction, and his calculations deserve little credit. His prediction rested on apparent tugs on the orbits of Uranus and Neptune. Johns Hopkins APL notes that those perturbations were not real, and that Pluto is far too small to have caused them anyway. The find came from Tombaugh’s patient search.

Pluto had even been caught on film before. Lowell staff later found that Thomas Gill had photographed it at the observatory in 1915, without anyone noticing, fifteen years before Tombaugh. The name Pluto, proposed by an 11-year-old Oxford girl, Venetia Burney, became official on May 1, 1930.