Tombaugh Discovers Pluto

Clyde William Tombaugh, a self-educated astronomer, discovered Pluto, which was then regarded as the ninth planet in Earth’s solar system.

Summary of Event

In 1920, Clyde William Tombaugh purchased a 2.2-inch (5.7-centimeter) telescope through the mail from the Sears, Roebuck catalog and taught himself the science of astronomy by reading everything he could on the subject. In 1924, he was impressed with Latimer Wilson’s Wilson, Latimer article “The Drift of Jupiter’s Markings” in Popular Astronomy. Wilson’s article included drawings he had made after observing the planet Jupiter with a homemade telescope. Tombaugh learned how to make telescopes through correspondence with Wilson. After Tombaugh built his telescope, he observed Jupiter and Mars, and in 1928 he recorded these observations in drawings. Fascinated with drawings of Mars released by the Lowell Observatory Lowell Observatory in Flagstaff, Arizona, and published in Popular Astronomy, Tombaugh sent his 1928 drawings to the observatory’s director, Vesto Melvin Slipher, Slipher, Vesto Melvin for some advice. [kw]Tombaugh Discovers Pluto (Feb. 18, 1930)
[kw]Pluto, Tombaugh Discovers (Feb. 18, 1930)
Pluto (planet);discovery
[g]United States;Feb. 18, 1930: Tombaugh Discovers Pluto[07550]
[c]Science and technology;Feb. 18, 1930: Tombaugh Discovers Pluto[07550]
[c]Astronomy;Feb. 18, 1930: Tombaugh Discovers Pluto[07550]
Tombaugh, Clyde William
Lowell, Percival

Clyde William Tombaugh.


The Lowell Observatory was founded by Percival Lowell in 1894. Lowell was fascinated with the maps of the “canals” of Mars Mars (planet), canals
Planets;Mars made popular by the Italian astronomer Giovanni Virginio Schiaparelli Schiaparelli, Giovanni Virginio from 1877 to 1888. Lowell’s observations extended the few dozen canals mapped by Schiaparelli to several hundred. He later popularized his speculations on Mars and alien civilizations in several books: Mars (1895), Mars and Its Canals (1906), The Evolution of Worlds (1909), and Mars as the Abode of Life (1908). Although Lowell’s theories excited the public and inspired science-fiction writers, astronomers were not convinced. In an attempt to improve his credibility, Lowell initiated a search for a ninth planet in Earth’s solar system.

Neptune, the eighth planet, had been discovered in 1846 based on the gravitational effect it was having on the orbit of its neighbor, Uranus. In the same way, the orbit of Neptune was different from what had been predicted, and this led astronomers Lowell and William Pickering Pickering, William to suspect a yet more distant planet. Lowell called this object “Planet X”; Pickering called it “Planet O.” Lowell reasoned that if he could predict the orbit of a ninth planet beyond Neptune and then find the planet, the discovery would enhance his professional status and thereby gain respect for his theory of Martian canals.

Lowell began searching with a special camera in 1905, confident of the planet’s location and brightness. In 1911, he obtained a new research tool, a Zeiss blink-microscope comparator, Blink-microscope comparator[Blink microscope comparator] to examine the photographic plates. With this device, two photographic plates of the same star region taken at different times are alternately seen or “blinked” in the viewer. Stellar objects, at their great distances, do not appear to move in the time between exposures. Closer objects, such as planets or asteroids, shift on the photographic plate and appear to “blink.”

After ten years of searching, Lowell became discouraged. Ironically, Planet X appeared on two separate photographic plates taken before his death in 1916. The planet was camouflaged by the Milky Way background between the constellations Taurus and Gemini. In 1919, Pickering, at the Mount Wilson Observatory, captured the planet on four different photographic plates yet failed to identify it.

In February, 1929, the Lowell Observatory resumed the quest for Planet X with the completion of a 13-inch (33-centimeter) telescope-camera. Tombaugh’s letter and drawings of 1928 could not have arrived at a better time. His observations and drawings caught the attention of Slipher, who was looking for a talented amateur to operate the new photographic telescope. When Slipher offered Tombaugh the job, he accepted.

Initially, Slipher told Tombaugh to search the Gemini region. It took about a week for Slipher and his brother, Edward, to blink the Gemini plates, without result. Disappointed, Slipher directed the research eastward through the zodiac and in June of 1929 asked Tombaugh to take over the task of plate blinking. Tombaugh found plate blinking tedious and was often distracted by other objects in the photographs.

Frustrated with this Herculean task, Tombaugh devised a technique for photographing a region of the zodiac when it was “at opposition”—that is, on the side of Earth opposite the Sun. Having done so, he noticed several things. At opposition, asteroids shifted about 0.28 inch (7 millimeters) per day. Neptune, being farther from Earth and from the asteroid belt, shifted less, about 0.08 inch (2 millimeters) per day. Tombaugh then reasoned that any undiscovered planet beyond Neptune ought to shift less than Neptune. If he could find such a planet, it would truly be Planet X.

In early 1930, Tombaugh resumed plate blinking, but the proximity to the Milky Way slowed his progress. It was then that Tombaugh realized that the Sliphers had blinked through the 1929 Gemini plates in only about a week. He suspected that the Sliphers, in rushing the job, had missed something. Therefore, on the basis of his earlier theory, he decided to rephotograph the region, this time near opposition. The first exposure of the Gemini region on January 21, 1930, was disturbed by wind gusts that shook the observatory, disturbing the telescope and blurring the image. He rephotographed this region on January 23 and 29 and began the tedious blink procedure.

Tombaugh retrieved the poor January 21 plate, compared it with the January 23 plate, and found Planet X exactly where it should be. Using a hand magnifier, he then compared the plates with another taken by a smaller camera. The object was in the same corresponding position on all three plates. Tombaugh then called Carl O. Lampland Lampland, Carl O. and Slipher to the blink comparator to confirm his find. Both agreed that the object could be Planet X, and Slipher asked Tombaugh to rephotograph the region as soon as possible. Based on photographs shot on February 18, 1930, Lampland, Slipher, and Tombaugh were able to confirm the presence of Planet X.


Slipher was aware of the impact of the discovery and carefully prepared for the public announcement and the questions that would result. The observatory’s reputation was in question over the Martian canal research, and the researchers had to be very sure not only of their data but also of the protocol involved in the announcement.

When the discovery became known, thousands of letters arrived suggesting names for the planet. Because planets were usually named after mythological deities, three names headed the list: Minerva, Pluto, and Cronus. The first person to propose the name Pluto outside the Lowell group seems to have been Venetia Burney, an eleven-year-old child living in Oxford, England. Pickering, who had predicted a trans-Neptunian planet in 1908, also suggested Pluto, the name of the Greek god of darkness, who was able at times to render himself invisible. Without doubt, all involved in the search would agree this quality of invisibility was appropriate for the new planet.

The Lowell group proposed the name Pluto to the American Astronomical Society and the Royal Society, and both societies accepted it unanimously in 1930. In 1931, the Associated Press declared the discovery of the planet Pluto to be one of the top news stories in the world for 1930.

From the mid-1990’s onward, new discoveries made possible by improvements in technology led to debates among astronomers concerning whether particular bodies in our solar system should be classified as planets. In 2006, the members of the International Astronomical Union voted to reclassify Pluto as a “dwarf planet,” reducing to eight the official number of bodies in the solar system defined as true planets. The decision was itself the source of controversy, and the debate concerning Pluto’s status continued. Pluto (planet);discovery

Further Reading

  • Freedman, David H. “When Is a Planet Not a Planet? Arguments for and Against Demoting Pluto.” Atlantic Monthly, February, 1998. Reviews the debate that arose in the mid-1990’s regarding the status of Pluto as a major planet, given new discoveries.
  • Hoyt, William G. Planet X and Pluto. Tucson: University of Arizona Press, 1980. Comprehensive volume documents the transition between Lowell’s work and Tombaugh’s. A good source for the interested reader.
  • Tombaugh, Clyde William. “The Discovery of Pluto: Some Generally Unknown Aspects of the Story.” Mercury 15, no. 3 (1986): 66-72; 15, no. 4 (1986): 98-102. Tombaugh describes his experiences on becoming an astronomer and the events leading to the discovery.
  • _______. “Reminiscences of the Discovery of Pluto.” Sky and Telescope 19 (1960): 264-270. Tombaugh’s own summary of the people and events surrounding the discovery.
  • Tombaugh, Clyde William, and Patrick Moore. Out of Darkness: The Planet Pluto. Harrisburg, Pa.: Stackpole Books, 1980. Provides a good review of all aspects of the search for planets and the personalities surrounding the events of the Pluto discovery. Accessible to the general reader.
  • Whyte, A. J. The Planet Pluto. New York: Pergamon Press, 1980. A relatively technical work for readers who wish to dig into the details of the discovery of Pluto. Provides an objective view of the process. Includes a historical review.

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