Clyde Tombaugh, Pluto finder

John Stanley
The Arizona Republic
Feb. 11, 2008 12:00 AM

Most people assume astronomer Clyde Tombaugh discovered Pluto by peering intently through a telescope. But that's not how it happened.

On a cold, dreary February afternoon almost 78 years ago, Tombaugh sat in the comfort of his office, examining pairs of photographic plates he had made a couple of weeks earlier.

He was 24 years old and had been working at Lowell Observatory in Flagstaff for just over a month. Before his hiring, he had been an unknown, largely self-educated amateur astronomer living in Kansas. After finding Pluto, though, his name burst upon the world like a supernova.

"(The discovery) hit me like a ton of bricks," Tombaugh said in a 1990 interview. "I knew in an instant I would become famous and my life would be changed forever."

Tombaugh was born in Illinois on Feb. 4, 1906, and moved with his family to Kansas in 1922. He was an excellent student and athlete in high school, interested in a variety of subjects.

But he became especially interested in geography, and one day he began to wonder about the geography of other planets. Tombaugh's uncle, an amateur astronomer, let Clyde borrow his telescope and lent him books on the subject.

Like countless backyard astronomers, Tombaugh quickly decided he needed a bigger telescope to see some of the things he had been reading about. Telescopes were expensive, though, so he decided to make his own.

The first one wasn't that great. The second was better, but it was his third telescope that led to his job in Flagstaff.

It was a 9-inch reflector, even today a respectable-size telescope for an amateur. With it, Tombaugh made a series of sketches of Jupiter and sent them to V.M. Slipher, director of Lowell Observatory.

Slipher was so impressed that he invited the young astronomer to Flagstaff and gave him an important task: to search for the elusive Planet X, the object postulated by Percival Lowell, the observatory's founder, in the early 1900s to explain certain anomalies in the orbit of Neptune.

'Tedious task'

At first, Tombaugh focused his search on the small section of sky where Lowell had predicted his Planet X should be found. When nothing turned up, Slipher told Tombaugh to search the entire zodiac (the belt of constellations through which the planets travel).

Tombaugh remembered that as "a horrendous, tedious task."

There were an average of 50,000 star images on each photographic plate, and every pair of plates, taken of the same section of sky, but several nights apart, had to be painstakingly examined with a device called the blink comparator, which would indicate, through the movement of one of the thousands of images, the presence of a planet. It was grueling work.

But on Feb. 18, 1930, at approximately 4 p.m., as Tombaugh was examining a section of sky in the constellation Gemini, he noticed a tiny point of light popping in and out of view.

Once he determined its direction and amount of motion, he knew he had found the long-sought Planet X.

"I'll never forget that night," he recalled in 1990. "The evening was cloudy, and there was no chance of examining the planet with one of the larger telescopes, so after having my dinner in Flagstaff, I went to the Orpheum Theater and saw Gary Cooper in The Virginian."

Slipher wanted to study the planet for a while before announcing the discovery, so the staff was sworn to secrecy. Tombaugh didn't even tell his parents, who learned about the discovery when a reporter called after the announcement to get their reaction.

Slipher revealed the discovery on March 13, 1930, on what would have been Percival Lowell's 75th birthday. Coincidentally, it was also the 149th anniversary of Wilhelm Herschel's discovery of the planet Uranus in 1781.

Later discoveries

The Associated Press voted the discovery one of the top news stories of 1930. Once the initial excitement of Pluto's discovery died down, Tombaugh went back to his survey. By 1943 he had photographed and examined nearly three-fourths of the sky. Although he didn't find any other planets, he discovered a remarkable assortment of astronomical objects: a globular cluster, six star clusters, about 1,800 variable stars, two comets, 700 asteroids, nearly 30,000 galaxies and the first known supercluster of galaxies.

In 1955, Tombaugh joined the faculty of New Mexico State University in Las Cruces. Long after his retirement in 1973, he remained active in the astronomical world. When he wasn't giving guest lectures, he often could be found in his backyard, still observing the heavens with the 9-inch reflector he made as a young man back in Kansas.

The Smithsonian wanted the reflector for its collection, but Tombaugh turned the Smithsonian down because, well into his 80s, he still was using it.

In 1978, astronomers discovered that Pluto had a moon, which they dubbed Charon. By observing Charon's motion, they finally could determine Pluto's size. It turned out to be much smaller than previously estimated - only 1,420 miles in diameter, about two-thirds the size of Earth's own moon.

Given its size, some astronomers wondered if it should be regarded as a planet. Tombaugh, who died in 1997, had always regarded that debate as nonsense. In 2006, the International Astronomical Union ruled that Pluto should be considered a dwarf planet.