Free Astronomy Magazine January-February 2020

HISTORY 5 T he 90 th anniversary of the discovery of Pluto gives us the opportunity to briefly review the events that led to the chance identification of that planet. We will focus our attention on the decisive months that saw Clyde Tombaugh com- plete that feat, which actually started a century and a half earlier. The chain of events that led to the discovery of Pluto began, indeed, on March 13, 1781, when the great sky observer Frederick William Herschel noticed what seemed to him to be a new comet, but was found over the following months to instead be the sev- enth planet of the Solar System, Uranus.