Using a measuring device, Tombaugh compared two photo plates that showed the same area of the sky but were taken a few nights apart. The pattern of the stars remained the same – but the planets and asteroids had wandered somewhat and jumped back and forth when comparing the two images.
Asteroids are much closer to Earth and therefore move much faster in the sky than a distant planet. Because of the small offset, Clyde Tombaugh immediately knew that the long search had finally led to success.
The Flagstaff observatory was set up by Percival Lowell, an industrialist from Boston, who was obsessed with the idea of another planet beyond Neptune.
The news of the successful discovery was therefore not published until three and a half weeks later, on March 13, 1930. On that day Percival Lowell, who had died 14 years earlier, would have been 75 years old.
The name of the planet was also chosen carefully: Pluto, with P and L at the beginning, Lowell’s initials.
It later turned out that Pluto had been photographed 15 years earlier, but was then overlooked in the tangle of stars. Percival Lowell would have experienced this discovery.
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