ALS, two pages, 8.5 x 11, New Mexico University, Department of Astronomy letterhead, May 14, 1980. Scientific letter to Harry Gould explaining the discovery of Pluto, in part: "While examining one pair of plates in eastern Gemini under the Blink–microscope–comparator on 18 February 1930, I encountered one faint star image that shifted position about 1/8th of an inch during the interval of 6 days on plates taken on 23 and 29 January 1930…The instant I spied the object jumping back and forth among the fixed small configurations of faint stars with rapidly alternating views of the two plates, the amount of shift instantly told me that the object was far beyond the orbit of Neptune. 'That's it!' I exclaimed to myself. It was an intense thrill…After checking with another plate taken on 21 January, the shift in position was consistent, as well as genuine. Planet X was the hypothetical name. It was named Pluto two months after the discovery." In fine condition. Accompanied by the original mailing envelope.Format: ALS