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Clyde William Tombaugh: The Young Astronomer Who Discovered Pluto

Updated on January 15, 2017


American astronomer Clyde Tombaugh is best known for his discovery of Pluto in the constellation Gemini in February 1930, which used to be designated as the ninth planet of the Solar System. Tombaugh stood on the shoulder of giants, but no one could take away from him the distinction of being the only astronomer who discovered a significant member of the Solar System—a planet—in the 20th century, and one of three human beings to discover a planet. He was the one who eventually found what many astronomers before him suspected as a member of the Solar System that was yet to be discovered—“Planet X.”

Tombaugh made other significant contributions to astronomy. He performed extensive his research and observations on the Moon and other known planets such as Venus, Mars, Saturn, and Jupiter. He also worked on the apparent distribution of various extragalactic nebulae. Clyde Tombaugh was also credited the discoveries of different galaxies and star clusters. One of the most significant findings that resulted from his search for “Planet X” was the “Great Perseus-Andromeda Stratum which consisted of 29,548 galaxies. This stratum also contained one nova, one globular cluster, two comets, five open star clusters, and about four thousand individual asteroids.

Family life and early interests

Tombaugh was born to a family of farmers residing in Streator, Illinois. Nevertheless, he showed interest in science, particularly astronomy early in childhood. He was an avid stargazer up to his teen years, an interest that was sparked when Clyde first glimpsed the heavens through his uncle’s telescope. The young Clyde realized soon enough that a telescope that bought from a store was not adequate for his needs. In order for him to observe the night sky better, he made his own equipment –a nine-inch reflector telescope—by grounding glass to make his own mirrors. For the body and other components of the telescope, he recycled parts from his father’s 1910 Buick and a cream separator. The specifications that Clyde followed were published in Popular Astronomy. He used the instrument that he made to learn more about the planets, stars, and other heavenly bodies.

Tombaugh received his undergraduate diploma from the University of Kansas in 1936. After his discovery of Pluto, he went to college, having been granted a scholarship. The young astronomer reported to Lowell during the summer. Once he earned his Bachelor of Arts in Astronomy degree, he went to work at the observatory full time. He also continued with his postgraduate studies and earned a master’s degree in astronomy in 1939.

Observatory where Clyde discovered Pluto
Observatory where Clyde discovered Pluto | Source

Work done as astronomer

In 1929, Clyde Tombaugh started working at the Lowell Observatory located in Flagstaff in Arizona. Tombaugh did not hold a college degree when he went to work at the observatory. Nevertheless, Lowell Observatory hired him because he showed great promise. Tombaugh produced drawings of Mars, the Red Planet and Jupiter, the biggest planet in the Solar System from the observations he made using the telescope that he constructed himself. He sent sketches over to Lowell. The amateur astronomer only wanted feedback and constructive critique of his observation techniques, but the authorities from Lowell had more than positive comments to give him. They made the choice to hire amateur astronomer Clyde Tombaugh for his skills. Director Vesto M. Slipher has faith in the promising young man.

Clyde Tombaugh’s first job at Lowell was assistant observer and he went to work with gusto and the full confidence of Lowell administrators despite his lack of formal training in astronomy. During the time that he worked at Lowell after receiving his college degree, he catalogued more than 30,000 objects, contributing important information to the astronomical community at that time. He left Lowell Observatory in 1946.

The work that Tombaugh was commissioned to do at Lowell Observatory was mainly concerned about finding a possible planet beyond the gas giant Jupiter. One of Clyde’s first assignments was to develop an observing procedure for finding distant planets. He was also tasked to bring the telescope online since it was relatively new. Many years before Tombaugh’s time, the search for a new planet was initiated by astronomer Percival Lowell, and team was brought together to continue the systematic study. Percival Lowell already predicted the possible location of the new planet based on available data on the orbital positions of Uranus and Neptune. Tombaugh and his associates worked non-stop for ten months before the discovery of Pluto.

After 1930, Tombaugh continued to engage in scientific pursuits primarily involving photographic search of the observable sky. He directed the team that conducted a research in Ecuador for Earth-orbiting debris. This yielded a null result, quite similar to the one he conducted in 1930 which led to the finding of Pluto.

While teaching at New Mexico State University, Tombaugh initiated and established a program related to NASA missions to the Red Planet and the outer planets of the Solar System. It was geared toward providing planetary images to be used in the Viking, Mariner, and Voyager missions. In 1950, Tombaugh was deeply involved in research on Mars. He predicted that the surface of the Red Planet would present with craters produced by asteroid impacts. Tombaugh’s predictions were confirmed by images taken by Mariner 4. He also participated in a mission assigned to the Jet Propulsion Laboratory with the objective of sending a mission to Pluto. Until old age, Tombaugh kept abreast of developments in NASA’s space program.

Three images of Pluto from the Hubble Space Telescope
Three images of Pluto from the Hubble Space Telescope | Source

The discovery of Pluto

After additional observations were conducted and the discovery was validated by the other astronomers, Pluto’s discovery was announced to the world in May 13, 1930. The date was significant in that it was the 75th birthday anniversary of Percival Lowell who started the search for a new planet, designated as “Planet X” in 1905. Lowell passed away in 1916. Tombaugh, who was then 24-years old, made the discovery at about four in the afternoon of February 18, 1930, one year after Lowell Observatory hired him as an assistant.

Clyde Tombaugh’s discovery of Pluto earned him the Jackson-Gwilt Medal in 1931. The Royal Astronomical Society bestowed this recognition onto him. The qualities that the Lowell administration recognized when they hired Tombaugh helped him attain the discovery of a lifetime. He used a 13-inch telescope to make photographs. The other important instrument at his disposal was the blink comparator, which detected the movement of celestial objects from comparisons of photographic plates. Pluto was revealed to Clyde Tombaugh while examining photographic plates dated January 23 and January 29, 1930.

Whereas Percival Lowell, his predecessor used calculations based on the perturbations and other relevant data culled from observations of the planet Uranus, Tombaugh used a different approach. He devised his own method. The premise of his technique was that “Planet X” would be moving retrograde at a very rapid rate enough for it to betray its real distance by parallax. He focused on photographing the area of the night sky where the new planet would possibly be at opposition. He spent his telescope time painstakingly taking images of the night sky using his own systematic approach involved photographing overlapping regions when these areas were on the meridian. The greatest displacement can be determined in this way since relative motions would appear to be perpendicular to the observer’s line of sight. Using this method, moving objects could be detected by comparing pairs of images that were taken several days apart.

Tombaugh’s perseverance and thoroughness definitely paid off. The position that Pluto was found was about six degrees from one of the predicted positions. “Lowell’s planet was located near Delta Geminorum. Later on, the object that Tombaugh discovered was renamed “Pluto.” It became evident that this planet was not massive or close enough to cause perturbations measured in Uranus, as Lowell posited years before. This confirmed the validity of Tombaugh’s new methodology. When Pluto was discovered, the available images were rather dull. It motivated Tombaugh to continue the search on the ecliptic for other planets.

Because of the discovery of Pluto, Tombaugh was motivated to find other possible objects in the darkness of the outer Solar System. By 1943, his thorough observations covered as much as 75% of the night sky that was visible from Lowell Observatory. Tombaugh’s work examined about 30,000 square degrees of the sky. It was comprehensive enough so that any planet the size of Earth would not be missed if they existed. His observations showed that no other planets were to be found—a null result. He was convinced that Pluto was the “last chance for a major planet in the Solar System. Later on, the Voyager and Pioneer missions confirmed Tombaugh’s findings that at the outer solar system, in locations of great distances from the sun, no large masses, which fall under the classification of planet is to be found.

Blink comparator used to discover the planet Pluto
Blink comparator used to discover the planet Pluto | Source

Pluto, the dwarf planet

Today, Pluto, which was named after the god of the underworld, has been demoted from planet to a new designation—as one of the dwarf planets in the Kuiper Belt that have established orbits around the sun. This new classification was endowed to Pluto in 2006.

Aside from its elongated orbit around the sun, many other unique conditions set Pluto aside. These conditions put into question whether it should be classified as a planet or not. For instance, it is too small compared to the other planets and seven natural satellites (moons) were bigger than Pluto was. Its volume is composed of 50% ice, which is similar to that of comets. Charon, the most proximal of the moons that orbit Pluto has a relatively huge mass that places the center of gravity of the duo outside Pluto. As such, Pluto was demoted as a dwarf planet. Now it joins the list that includes Eris, Ceres, Makemake, and Haumea.

Tombaugh video

Other positions held

From 1955-1973, Clyde Tombaugh conducted researches and taught astronomy at the New Mexico State University. He was instrumental in the establishment of the astronomy research program of the said university first as part of the physical sciences laboratory and then as full professor of astronomy and earth science. He played such a pivotal role in the astronomy program that the observatory, which was built by the university in 1972, was dedicated to him.

Tombaugh was part of the missile development program of the United States after the Second World War at New Mexico’s White Sands Proving Grounds. In 1946, His work at the Ballistics Research Laboratory as optical physicist and astronomer was centered on determining characteristics, optical tracking systems, and flight paths of rockets. Prior to that, during the war, he served as a US Navy navigation instructor to help in the war effort. He was also part of the faculty of Arizona State College, teaching navigation. Tombaugh was an esteemed faculty member of the University of California in Los Angeles as well.

Clyde Tombaugh retired in 1973. He spent his later years teaching astronomy and geology, and continued to inspire new generations with his enthusiasm for science and discovery. Tombaugh also wrote a book about the discovery of Pluto with Patrick Moore in 1980. It was entitles, “Out of the Darkness: The Planet Pluto. Many years before, in 1959, Tombaugh wrote a book entitled, “The Search for Small Natural Earth Satellites.”

Excellent bio on Clyde Tombaugh - I have this book in my collection

Public promotion of science and exploration

The proud and dedicated astronomer who discovered Pluto made a huge combination in the promotion of the sciences to the public. Aside from writing a book about his major discovery in 1930, he conducted a tour of Northern America with Patricia Tombaugh, his wife. The public lectures were intended to raise funds and help improve the opportunities available to young scientists at that time. His fundraising efforts sought additional funding for the Clyde W. Tombaugh Scholars Fellowship, which he supported throughout his life.

Tombaugh was generous with his time and shared insights with amateur astronomers who sought his advice. He was very accessible to young astronomers who received his detailed response to their queries written in long hand. He continued to travel extensively, interacting with people who aspire for a career in astronomy until his health began to fail him. He was a pleasant and open personality with a contagious sense of humor. Tombaugh never grew tired of telling the story about his momentous discovery of Pluto.

Clyde William Tombaugh died peacefully in his home on January 17, 1997. He was 90 years old.


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    • Larry Rankin profile image

      Larry Rankin 2 years ago from Oklahoma

      Very interesting. Can't believe Pluto isn't a planet anymore, lol.