Clyde Tombaugh

Lore & Background

Tombaugh's interest in astronomy began when he visited Yerkes Observatory at age 12 in 1918. His family's poor finances forced him to drop out of high school for a year to help on the farm, and a hailstorm in 1928 ruined plans for college. He built several telescopes starting in 1926, digging a pit to test mirrors, and sent drawings of Jupiter and Mars to Lowell Observatory, which offered him a job in 1929. At Lowell, he was tasked with a systematic search for a trans-Neptunian planet (Planet X) predicted by Percival Lowell. Using a 13-inch astrograph and blink comparator, he discovered Pluto on February 18, 1930, using images taken the previous month. The name Pluto was proposed by 11-year-old Venetia Burney and officially adopted on May 1, 1930. Following the discovery, it was recognized that Pluto was not massive enough to be the expected ninth planet, and Tombaugh searched for additional trans-Neptunian objects for years; finding none, he concluded Pluto was a planet. The reclassification of Pluto as a dwarf planet occurred on August 24, 2006, after the discovery of Eris. Tombaugh also discovered hundreds of asteroids, variable stars, star clusters, galaxy clusters, and a galaxy supercluster, and may have discovered the Perseus–Pegasus Filament in 1936.

Reader's Guide

Clyde Tombaugh's discovery of Pluto in 1930 was the first detection of what would eventually be recognized as the Kuiper belt, a region of icy bodies beyond Neptune. His systematic search for a trans-Neptunian planet, using photographic plates and a blink comparator, demonstrated the power of patient observation. Although Pluto was initially classified as the ninth planet, later discoveries of other Kuiper belt objects, including Eris, led the International Astronomical Union to reclassify Pluto as a dwarf planet in 2006. Tombaugh's widow noted he would have accepted the change as a scientist. Hal Levison remarked that Tombaugh 'discovered the Kuiper Belt. That's a helluva lot more interesting than the ninth planet.' Beyond Pluto, Tombaugh contributed to astronomy by discovering hundreds of asteroids, variable stars, and galaxy clusters, and by leading the Planetary Patrol project at New Mexico State University, which helped determine Mercury's rotation period and monitored Jupiter's Great Red Spot. He also reported seeing unidentified flying objects in 1949 near Las Cruces, New Mexico.

Did You Know?

• The name Pluto was proposed by an 11-year-old English girl, Venetia Burney.
• Tombaugh may have discovered the Perseus–Pegasus Filament, a galaxy filament roughly a billion light years in diameter.

From Farm to Observatory: A Self-Taught Path

Clyde William Tombaugh entered the world on February 4, 1906, in Streator, Illinois, as the eldest of six children born to Muron Dealvo Tombaugh, a Pennsylvania Dutch farmer, and Adella Pearl Chritton. His fascination with the night sky ignited at age twelve during a 1918 visit to the Yerkes Observatory in Wisconsin, and was further nurtured by his uncle Lee, an amateur astronomer who shared his small refractor telescope and a collection of astronomy books. Financial hardship forced the family to relocate to a farm in Burdett, Kansas, in 1922, and young Clyde temporarily abandoned school to help prepare crops. A hailstorm in the summer of 1928 destroyed the family's harvest and derailed his college ambitions. Yet in 1924, an article in Popular Astronomy depicting sketches of Jupiter's surface convinced him he needed his own instrument. Beginning in 1926, he ground lenses and mirrors by hand, constructing multiple telescopes from spare parts. To test his mirrors under stable conditions, he excavated a pit twenty-four feet long, eight feet deep, and seven feet wide using nothing but a pick and shovel—a space that doubled as the family's root cellar and storm shelter. His hand-drawn observations of Jupiter and Mars eventually reached Lowell Observatory, which extended him a position.

The Blink Comparator and a New World

At Lowell Observatory in Flagstaff, Arizona, Tombaugh was tasked with a systematic search for a trans-Neptunian body that Percival Lowell had predicted based on calculations by Elizabeth Williams and William Pickering. Beginning April 6, 1929, he used the observatory's thirteen-inch astrograph to photograph the same stretch of sky on successive nights. He then compared pairs of images with a blink comparator, a device that rapidly alternated between two photographs so that any moving object would appear to jump while background stars held still. On Tuesday, February 18, 1930, examining plates taken the previous month, he spotted exactly such a wandering point near the region Lowell had predicted. Follow-up observations confirmed an orbit well beyond Neptune, eliminating the asteroid classification. Three names were floated—Minerva, Cronus, and Pluto. Minerva was already claimed by an asteroid, and Cronus was dropped because a widely disliked astronomer who championed it might falsely claim credit. Pluto, proposed by an eleven-year-old English girl named Venetia Burney, carried the advantage that its first two letters matched Lowell's initials and evoked the Roman god who could render himself invisible. Both the American Astronomical Society and the Royal Astronomical Society approved the name unanimously, and it was officially adopted on May 1, 1930.

A Broader Sky: Contributions Beyond One Discovery

During his tenure at Lowell Observatory from 1929 to 1945, Tombaugh's work extended far beyond a single famous detection. He identified hundreds of asteroids and catalogued a variety of star clusters, galaxies, and variable stars, making substantial contributions to the broader mapping of the night sky. After leaving Lowell, he earned both a bachelor's and a master's degree in astronomy from the University of Kansas in 1936 and 1938, respectively. He then joined the faculty at New Mexico State University, where he directed the Planetary Patrol project. Through that program, his team helped pin down Mercury's rotation period, kept watch on Jupiter's Great Red Spot, and developed novel photographic techniques aimed at finding new satellites. Even after his 1973 retirement, Tombaugh never set down his tools. He continued building telescopes, lending his expertise to ongoing observational programs, and championing the continued scientific study of unidentified flying objects. His career thus spanned from a self-taught farm boy grinding mirrors in a Kansas pit to a university professor shaping planetary science for decades.

A Shifting Sky: Legacy and Reclassification

For more than seventy years, Pluto held its place as the ninth planet of the Solar System, a classification that stood from its 1930 discovery until the mid-2000s. Yet even in the years immediately following the find, some astronomers noted that the object was not massive enough to match the expected ninth planet and speculated it might represent the first member of an entirely new class of body. Tombaugh himself spent years searching for additional trans-Neptunian objects, but the absence of further discoveries led him to conclude that Pluto was indeed a planet. The minority view that it was not a true planet persisted until the late 1990s, when a growing population of similar icy bodies in the same region of space revealed that Pluto did not orbit in isolation. The discovery of Eris, a body more massive than Pluto, finally prompted the International Astronomical Union to reclassify Pluto as a dwarf planet on August 24, 2006, reducing the Solar System to eight planets. Tombaugh's widow Patricia noted that he might have felt some disappointment at the change, given that he had spent his life identifying the object as a world in its own right.

Gallery