Editing Planets beyond Neptune (section)

=== Discovery of Pluto ===
{{main|Pluto}}
[[File:Clyde W. Tombaugh.jpeg|thumb|right|upright|Clyde William Tombaugh]]
Lowell's sudden death in 1916 temporarily halted the search for Planet&nbsp;X. Failing to find the planet, according to one friend, "virtually killed him".<ref>Croswell (1997), p. 49.</ref> Lowell's widow, Constance, engaged in a legal battle with the observatory over Lowell's legacy which halted the search for Planet&nbsp;X for several years.<ref name=cros2>Croswell (1997), pp. 32–55.</ref> In 1925, the observatory obtained glass discs for a new {{convert|13|in|cm|abbr=on}} wide-field telescope to continue the search, constructed with funds from [[Abbott Lawrence Lowell]],<ref>{{Cite web |last=Schindler |first=Kevin |date=2015-05-14 |title=Percival Lowell's three early searches for Planet X |url=https://www.astronomy.com/science/percival-lowells-three-early-searches-for-planet-x/ |access-date=2024-09-11 |website=Astronomy Magazine |language=en-US}}</ref> Percival's brother.<ref name=tombaugh/> In 1929 the observatory's director, [[Vesto Melvin Slipher]], summarily handed the job of locating the planet to [[Clyde Tombaugh]], a 22-year-old Kansas farm boy who had only just arrived at the Lowell Observatory after Slipher had been impressed by a sample of his astronomical drawings.<ref name=cros2/>

Tombaugh's task was to systematically capture sections of the night sky in pairs of images. Each image in a pair was taken two weeks apart. He then placed both images of each section in a machine called a [[blink comparator]], which by exchanging images quickly created a [[Time-lapse photography|time lapse]] illusion of the movement of any planetary body. To reduce the chances that a faster-moving (and thus closer) object be mistaken for the new planet, Tombaugh imaged each region near its opposition point, 180 degrees from the Sun, where the [[apparent retrograde motion]] for objects beyond Earth's orbit is at its strongest. He also took a third image as a control to eliminate any false results caused by defects in an individual plate. Tombaugh decided to image the entire zodiac, rather than focus on those regions suggested by Lowell.<ref name=tombaugh/>

[[File:Pluto discovery plates.png|thumb|left|Discovery photographs of Pluto]]
By the beginning of 1930, Tombaugh's search had reached the constellation of Gemini. On 18 February 1930, after searching for nearly a year and examining nearly 2&nbsp;million stars, Tombaugh discovered a moving object on photographic plates taken on 23 January and 29 January of that year.<ref>Tombaugh (1946), p. 79</ref> A lesser-quality photograph taken on January 21 confirmed the movement.<ref name=cros2/> Upon confirmation, Tombaugh walked into Slipher's office and declared, "Doctor Slipher, I have found your Planet&nbsp;X."<ref name=cros2/> The object was just six degrees from one of two locations for Planet&nbsp;X  Lowell had suggested; thus it seemed he had at last been vindicated.<ref name=cros2/> After the observatory obtained further confirmatory photographs, news of the discovery was telegraphed to the [[Harvard College Observatory]] on March 13, 1930. The new object was later [[Precovery|precovered]] on photographs dating back to 19 March 1915.<ref name="Hoyt">{{Cite journal |last=Hoyt |first=William Graves |date=December 1976 |title=W. H. Pickering's Planetary Predictions and the Discovery of Pluto |journal=Isis |language=en |volume=67 |issue=4 |pages=551–564 |doi=10.1086/351668 |issn=0021-1753 |jstor=230561 |pmid=794024 |s2cid=26512655}} p. 563.</ref> The decision to name the object ''Pluto'' was intended in part to honour Percival Lowell, as his initials made up the word's first two letters.<ref>{{cite web|title= NASA's Solar System Exploration: Multimedia: Gallery: Pluto's Symbol |url= http://sse.jpl.nasa.gov/multimedia/display.cfm?IM_ID=263 |publisher=NASA |access-date=2007-03-25 |url-status=dead |archive-url= https://web.archive.org/web/20061001015053/http://sse.jpl.nasa.gov/multimedia/display.cfm?IM_ID=263 |archive-date=2006-10-01 }}</ref> After discovering Pluto, Tombaugh continued to search the ecliptic for other distant objects. He found hundreds of [[variable star]]s and [[Clyde Tombaugh#Asteroids discovered|asteroids]], as well as two [[comet]]s, but no further planets.<ref>{{cite web|title=Clyde W. Tombaugh|publisher=New Mexico Museum of Space History |url=http://www.nmspacemuseum.org/halloffame/detail.php?id=51|access-date=2008-06-29}}</ref>