Free Astronomy Magazine January-February 2020

15 JANUARY-FEBRUARY 2020 HISTORY by Lowell and, having found it right in the region of sky indicated by Lowell, its dis- covery may have only been an incredible coincidence. In the days following the first direct observation, other plates were taken, the com- putation of the orbit was im- proved, and the team de- cided how and when to an- nounce the discovery. The date of March 13 was chosen because it coincided with the 149 th anniversary of the dis- covery of Uranus (which had, in a certain sense, instigated the subsequent events) and with the 75 th anniversary of Lowell’s birth. On March 13, 1930, Tombaugh and Vasto Slipher sent a telegram to the already famous Harlow Shapley, director of the Har- vard College Observatory, in which, in addition to announcing the dis- covery, every relevant piece of information was provided. From that day and for an- other 76 years, our Solar System would have nine planets. On the first day of May 1930, the Flagstaff Observatory team chose the name of the new planet from over one thousand pro- posals received. The choice fell on Pluto, the god of the underworld, proposed by a very young schoolgirl in Oxford. Until recently, the story of Pluto after 1930 was only a little better known than the story before. For almost half a century after its discovery, Pluto remained a perfect un- known: except for the oddly inclined orbit and its 2:3 resonance with Neptune, and for the facts of being small and cold, nothing else was known. Diameter, mass and albedo were only imaginable. In 1978, however, there was a turning point with the discov- ery of Pluto’s largest satellite, Charon, which allowed the mass and other major physical properties to be calculated more accurately. The rest is recent history. In 2006, after long and heated discussions, the International Astronomical Union down- graded Pluto to the rank of dwarf planet. In 2015, NASA’s New Horizons mission rev- olutionized our vision of Pluto and its satel- lite system. The reader interested in the results of this mission may find it useful to read our articles “Astonishing Pluto” and “The new face of Pluto’s system” . ! I n these two photos, the af- fection of an aged Clyde Tombaugh for as- tronomy is still alive. He died on 17 January 1997, and part of his ashes will travel beyond the Solar System on board the New Horizons probe.