(This article originally appeared in a 1990’s STAR newsletter.)

Clyde Tombaugh’s Pluto
By Sean League

At four o’clock on 18 February 1930 an unbelievable amount of patience and effort paid off. A new planet was discovered. The planet, Pluto, was at last discovered by Clyde Tombaugh.

Clyde Tombaugh was born on 4 February 1906 on a farm near Streator, Illinois. He had four brothers and sisters, all of which were younger. Clyde and his family often visited his Uncle Lee and his family on weekends. His Uncle Lee, who lived nearby, was an amateur astronomer and had a three inch telescope. Due to the smallness of his uncle’s scope he could only see Jupiter, its moons, Saturn, Venus and the Moon, but it got Clyde interested in astronomy. Besides his Uncle Clyde letting him borrow the scope, he had a small astronomy book, which he memorized. Galileo, Herschel, and Lowell, who discovered the moons of Jupiter, Uranus, and almost Pluto, respectively, became his heroes. < Near the end of WWI there was a shortage of manpower and Clyde was needed to work on the farm. Times became a little worse so Clyde's family decided to try their luck farming in Kansas. In 1922 they moved to Burdett, Kansas, where 250 acres of farming needed to be done by fall if they were to survive. As a result, Clyde had to take off a year of high school. Before they moved, Clyde's father and his Uncle Lee bought a 2.25-inch achromatic telescope (light correcting and even though it is smaller than the 3-inch it was probably better). Uncle Lee gave it to Clyde's father when they moved. Kansas had much clearer weather than Illinois, so Clyde spent many more hours observing. Clyde graduated from high school in 1925, but his family did not have the money for college. In addition, crop yields were low and Clyde was needed on the farm. It looked as though Clyde would end up with a regular job for the rest of his life, although he tried to find something that was different. Clyde knew that he would like going to college and working at a university, but he also knew that it was out of his reach. By 1925, Clyde had become obsessed with getting a larger telescope, but buying one was out of the question. He was subscribing to an astronomy magazine and saw an article about someone who had made his own 11-inch. This got him excited and he wrote a letter to the man in the article. The man told him where to get the supplies (glass disks, grit, rouge, etc.) and where to get direction on how to make a telescope. Clyde did his best, but when it came to checking the curve of the mirror the Foucault test was inaccurate. This was due to the fact that he had no basement and had to perform the test inside his heated house (because heat rises the movement of the air interferes with the test). After he unsuccessfully tried to silver his polished mirror, he sent it off to Napoleon Carreau who said that it had a poor figure. The finished telescope was better than anything he had before, but he knew it could be better. The only problem was he needed better test conditions. His father decided that they needed an under ground safe haven from tornados and a place for storage, so they built an 8 by 7 by 24 foot `basement'. Clyde convinced his father to make the basement longer in order to test his mirror. With improved conditions he ground a new 7-inch mirror for his Uncle Lee. He then ground a 9-inch for himself. He finished it with only 24 hours of work! When he sent it to Carreau for silvering, he was impressed by the figure. With this new mirror he began making detailed drawings of Mars and Jupiter. He decided to send them to Lowell Observatory in Flagstaff, Arizona. About the same time, he received a letter from Carreau asking him to help grind mirrors for his business. He would have said yes if it had not been for Lowell Observatories' need for a good amateur to work their new 13-inch. When the observatory asked him if he would come to Flagstaff and work, it was a chance he could not pass up. He used some money he had made by running his neighbors combines for a one-way train ticket. While Clyde was a small boy, a man named Percival Lowell was looking for what is now Pluto. In the early 1800's it had been predicted that there was another planet beyond Uranus. This was due to the fact that there were unexplained perturbations in Uranus's orbit. Using calculus and some pretty tough math, two men, Leverrier (French) and Adams (English), unaware of each other at the time, predicted where the planet should be and what its period of revolution (around Sol) should be. Through a "comedy of errors" the English astronomers did not follow up on Adams prediction and Leverrier convinced a German astronomer to look. The German astronomer, Galle, (and his assistants) found the planet within one degree of where it was supposed to be. Finding Neptune did not, however, solve the whole problem, because it was noticed that Neptune also had a wobble. Lowell predicted that there should be yet another planet. Although he tried searching for it, he died in 1916 with no results. At the time of Clyde's invitation, Lowell Observatory was, for the most part, run by Dr. Lampland and Dr. V. M. Slipher. When Clyde arrived at Flagstaff, he was met by Dr. Slipher. He was then taken to the observatory and to where he would be staying. After a frightening drive up the snow covered mountain, Clyde found it very strange and unnerving to be staying at an altitude of 7000 feet - quite a change from flat Kansas. The 13-inch scope was built, but the lenses had not come in yet. The first thing he was assigned to do was chores, some of which were knocking snow off the domes, throwing huge logs into the administrative building's furnace, and showing tourists around. On 11 February 1929, the 13-inch lens arrived. When it came to putting the lens in, the mood was very excited and tense. As Clyde said "A screwdriver was never more cautiously used." The lens was then VERY CAREFULLY bolted on to the upper end of 5-foot-long tube. After some initial photographic tests, it was found that the lens was of very good quality. The photographic search for `Pluto' was finally restarted on 6 April 1929. The search was conducted by taking several photos of the same region over several nights and comparing them on the blink-comparator. The blink-comparator was a device that would take a plate (large negative) and blink it on for a second and then switch to the next negative. When the two plates are blinked, they should look exactly the same and if any `stars' are in a different locations you have discovered a planet, a comet, or an asteroid (the way to tell the difference between one of these is a little more difficult). Over the next several years Clyde would spend 7000 hours examining 90 million individual stars through the blink-comparator. On 18 February 1930 about four in the afternoon, Clyde had almost finished covering a fourth of a Gemini (from the constellation Gemini) plate. After looking at a field containing the star Delta Gemini he moved on to the next small field. Then he noticed a fifteenth magnitude `star' blinking in and out on two of the plates. He quickly measured the distance the `two' objects on the two plate had moved relative to the background stars. He found that the image had moved westward, which is retrograde motion. This is caused by Earth moving around Sol. It causes other planets to appear to move `backward' or, in other words, retrograde motion. The distance he measured between the two images was a small enough parallactic shift to indicate that it was beyond Neptune's orbit. A parallactic shift is caused by Earth moving around Sol. It is the same as the shift you get when you shut one eye and then the other. Clyde "was in the most excited state of mind" that he had ever been in his life. He checked and made absolutely sure he was right before running down the hall to get Dr. Slipher and Dr. Lampland. To make absolutely sure, they took more pictures and followed it for another three weeks before telling anyone. When they finally released the news, it made headlines everywhere and ensured the survival of Lowell Observatory, which had been in financial difficulty for some time. After a few weeks it was found that Pluto could not have enough mass to have a gravitational effect on Neptune (the reason for the search). This can only mean that there is still another planet or something out there that we have not found yet (the so called planet X or planet P as called by William Pickering). Since its discovery it has been found that Pluto has a moon, Charon, which is about half its size (more like a double-planet than a planet-moon pair). Until a few months ago Pluto's and Charon's images had never been resolved as separate images. This was done by the Hubble Space Telescope, which also showed a disk for both Pluto and Charon (even though the Hubble is "broken" it still out does anything on Earth). Clyde Tombaugh is still alive and often gives talks about Pluto and how he discovered it. He is a very interesting and intelligent man and I encourage you, if you ever get the chance, to hear him give one of his inspiring talks. Bibliography
Tombaugh, Clyde; Moore, Patrick; Christy, James. Out of the Darkness. Harrisburg, Pa. Stackpole Books, 1980.
Moore, Patrick. The International Encyclopedia of Astronomy. 1987 ed.