Photo: the two photographic glass plates Clyde Tombaugh compared to each other to ultimately discover Pluto.
February 18, 1930: Clyde Tombaugh discovers Pluto
Clyde Tombaugh at the guide scope for the 13″ astrograph used to discover Pluto | Lowell Observatory Archives
On February 18, 1930, newly-minted, 24-year-old astronomer Clyde Tombaugh spotted what was then referred to as Planet X on plates taken on the 23rd and 29th of January in the same year. Tombaugh would name the planet Pluto, as suggested in a letter from an 11-year-old English girl named Venetia Burney.
The process leading up to the historic discovery was a laborious one: Tombaugh would photograph the same section of the sky several days apart and use a Zeiss Blink Comparator to detect the motion of a nearby planet against the more distant “fixed” stars. Considering the search for what Percival Lowell referred to as Planet X had spanned nearly three decades (with some interruptions in between), the fact that Tombaugh discovered it in a matter of months was particularly impressive. The discovery was announced to the public March 13, 1930 — Percival Lowell’s birthday.
February 19, 1473: Copernicus is born
A portrait of Copernicus | Getty Images/GraphicaArtis