CLYDE TOMBAUGH

From Kansas to Pluto

Astronomers used to be thought of as lone explorers sitting at the telescope night after night, just waiting to make the next big discovery. Nowadays, they work in teams, sometimes in multinational collaborations, each one contributing to the work of discovering and explaining the cosmos. In 1930, however, there was a lone explorer sitting at a special instrument at Lowell Observatory in Flagstaff, Arizona. His job was to take plates of the sky and then compare them to see if anything moved between exposures. His name was Clyde Tombaugh, and his painstaking work resulted in the discovery of Pluto, orbiting in the distant reaches of the solar system.

The Man Behind the Discovery

Clyde Tombaugh was born in Illinois in 1906, the son of a farmer and his wife. He and his father were both avid amateur observers, and Clyde often described himself as a young man building telescopes out of whatever he could find and grinding his own lenses. He wanted to attend college, but his family didn’t have the money to send him, so he continued building telescopes and making observations of Mars and Jupiter. He sent some of his best ones to Lowell Observatory, hoping to get some advice from the observatory staffers. Much to his surprise, they contacted him with a short-term job offer. They needed an amateur astronomer to help operate their telescope. Tombaugh took the train to Flagstaff in mid-January 1928 and was met by astronomer Vesto Slipher (1875–1969). The job started right there, and Tombaugh ended up staying for thirteen years.

In 1928 he began doing planet searches. In particular, the director wanted him to find the mysterious Planet X that might exist out beyond Neptune. The project was spurred by Percival Lowell, who had founded the observatory but died in 1916. Lowell had been extremely interested in finding this unknown world and started a search program about a decade before he died.

Once at Lowell Observatory, Tombaugh began using an astrograph to take photographs of sections of the sky where the hidden planet was thought to be. He’d take a set aimed at one place one night, and then a few nights later, he’d take another set of the same place. He would then compare them using what’s called a blink comparator. This instrument allowed him to switch very quickly between one image and another to see if anything moved. If it did, Tombaugh made a note of it. It took him weeks to examine each plate that he took. Eventually, he found a very dim object that appeared to jump from frame to frame in a set of observations he’d made a week earlier. Further observations allowed an orbit to be calculated, which turned out to lie beyond Neptune. Tombaugh made his discovery on February 18, 1930. When the discovery was announced on March 13, it electrified the world. Pluto was the first planet to be discovered since Neptune had been found in 1846, and it was the first to be found by an American astronomer. The new planet was named Pluto, and it made Tombaugh famous.

During his search for the planet, Tombaugh also discovered more than 800 asteroids, hundreds of variable stars, and photographed such objects as star and galaxy clusters. In later years, Tombaugh went on to become a college teacher in Flagstaff and at the University of California at Los Angeles. He then worked at the White Sands Missile Range in the Ballistics Research Laboratory before taking a teaching position at New Mexico State University in 1955. There he spent time building up the astronomy department and its facilities before retiring to a life of stargazing and public lectures about his work. Clyde Tombaugh died in 1997, leaving behind a solid legacy as an observational astronomer. In his honor, some of his ashes are on their way to Pluto as part of the New Horizons mission to the outer solar system.