Free Astronomy Magazine January-February 2020

11 JANUARY-FEBRUARY 2020 HISTORY ity, Planet X was present near the edge of two plates ex- posed on March 19 and April 7, 1915, but no one noticed it. Without the spur of Lowell and with the First World War on its doorstep, the Flagstaff Observatory experienced a long, dark period. Planet X hunting resumed in 1928 at the initiative of a nephew of Lowell, the politi- cian and businessman Roger Lowell Putnam, and with the help of Lampland and the Slipher brothers. Thanks to new funds, the observatory was equipped with a 13-inch astrograph and the research team was expanded with the hiring of a young amateur as- tronomer from Kansas, Clyde Tombaugh, whose main task would be to examine the plates with the blink. In the spring of 1929, the enterpris- ing amateur astronomer (he will become an astronomer seven years later) began his activity. According to Lowell’s calculations, at that time, Planet X would have been hidden in the region of sky around Delta Geminorum. Before that sea- son ended, several hundred plates had al- ready been exposed, and on two of them, centered precisely on that star, the planet was there. Perhaps due to a hasty compari- son in the blink, or more probably because at the time the planet was almost station- ary, it happened that no one noticed its presence. The new astrograph was so powerful (and proba- bly the plates so sensi- tive) that a problem be- gan to emerge: each image contained thou- sands of stars, and on each plate appeared up to dozens of objects flickering in the blink. Most were known aster- oids, but there were also unknown asteroids, sta- tionary meteors, defects C lyde Tombaugh near the eyepiece of the 13-inch re- fractor used to discover Pluto. [Lowell Observa- tory Archives] A plate ex- posed in De- cember 1925 in which Pluto ap- pears. The planet had been pho- tographed on several occasions since 1909 but was first identi- fied only in Feb- ruary 1930.