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Interview of Clyde Tombaugh by John Lutnes in 1969, Niels Bohr Library & Archives, American Institute of Physics, College Park, MD USA, www.aip.org/history-programs/niels-bohr-library/oral-histories/4916
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Discovery as an amateur astronomer of the planet Pluto (Percival Lowell’s planet X) at the Lowell Observatory in 1930. Events leading up to the search and Tombaugh’s involvement. Comments on the history of the Lowell Observatory as Percival Lowell’s privately built and owned planetary observatory. Also prominently mentioned is: V. M. Slipher.
I should have referred to these large 14- and 17-inch plates like you see in the plate holder or not. Leave that out.
There’s something wrong here. I think we should go ahead and record right now, so do you want to talk about your (???). Just go ahead.
In western Kansas in my high school days I did a lot of observing with a little homemade telescope and so on, and it was really a pretty good one. I made drawings of Mars and Jupiter, sent them out to Lowell Observatory, which I knew specialized in planetary astronomy; and Dr. V.M. Slipher appeared to be interested enough that he wrote me and asked some questions and finally ended up asking me if I’d like to come out on trial. They had a new telescope about ready to operate, and that I might be interested in observing. So I came out and found out that they were going to make an attempt to find Dr. Lowell’s predicted Planet X, and the telescope wasn’t quite completed when I arrived, and I remember the day when the lens came. (???) came to see if it was all in one piece. And then in the spring of 1929, the telescope was ready to go and so my work consisted of taking these large 14 X 17 inch negatives by enlarged exposure of the regions along the Cephidis, where the planets are expected to be found. And then I would examine these plates under the Bink microscope a pair of the same region, with the plates taken a few nights apart, which were suitable for showing planetary motion, which was the test that it was a planet.
This went on for almost a year and finally one day in February, 1930, I came upon the region where the Pluto images were, and it was a tremendous thrill, and I spent about three-quarters of an hour checking on some other plates, and they seemed to be real. There weren’t defects of some kind. So then I went down and told Dr. Slipher, and of course he was tremendously excited, and so we watched it for a few weeks before it was announced to the world. Now, these large plates are quite laborious to examine. In the (???) regions, the stars are very rich indeed. Some of the plates in the Scorpius/Sagittarius regions were running right around over a million stars apiece, and you can only do a few square inches a day as a hard day’s work, and it takes about a month to do about one pair thoroughly. Now, in the thinner regions one can get over a pair in about three days. This search went on for about 530 pairs over a period of about 14 years. I covered three-fourths of the entire sky, everything down to the 17th magnitude. And 19 million stars passed in review under the Bink microscope for a total of about $7000.
Is that right? $7000.
Yes, $7000. One of the things early… And of course it was a real adventure for me to search through the skies and know what was there, and I would sometimes work as much as seven hours a day at this Blink (?) microscope comparing (???). But as the years went by, I found I could do less and less. Somehow it was so tedious that I finally could not do more than about two hours a day. But then the war came along, World War II, and then I was on some other duties, and I covered about three-fourths of the sky and that was the end of that. But we essentially completed the search when that took place.
How old were you when you actually discovered Pluto?
I was 24 years old.
Twenty-four years old. You said you were excited.
Yes, very excited.
Could you tell a little more in detail just exactly what happened? I think it would be interesting.
Well, the (???) of course are little black (???) because we examined the original negatives, and we had studied the shifts and so on theoretically about what to expect, and pairs of plates after plates had gone on; and finally this one day — it was about four o’clock in the afternoon — I came upon these images and the shift was right, the images looked real, and I knew it wasn’t a comet and it wasn’t something else — it had to be a planet beyond the orbit of Neptune. And it was a tremendous thrill, because I thought at the time: “That’s it!” Whereas some others: in the case of Uranus, Herschel thought he had found a comet first and so on. So I had the full benefit of the intense thrill of having seen another planet for the first time, and then we continued observations of it to study its motion, be absolutely sure and when it was expected and finally it was announced to the world on the 13th of March, 1930. Now, the 13th of March was about the time we were pretty certain of it, and also it coincided with Lowell’s birthday, and also the 149th anniversary of the discovery of Uranus by Herschel. So it was a sort of a special day.
Yes, I see. Now, I understand that Lowell — he predicted this. Is this right?
Yes. There were some leftover residuals, as they called them, from Uranus at Neptune that didn’t seem to fully satisfy, and it appeared for a while that maybe this was going to be a sort of repeat performance as in the case of the discovery of Neptune, which was predicted theoretically by gravitational disturbances of Uranus. But as it turned out later, Lowell realized that this was very very small, and he wasn’t perhaps absolutely sure about it, but at least the mathematics was correct. And it later turned out that Pluto was too small to have been due to this effect. He, assuming these residuals were real, would have required a mass of about seven times that of the earth. And as it turns out now, we know that Pluto has a mass of the order of about 1 2/10 that of the earth and that the discovery really was due to a systematic photographic survey instead. Nevertheless it was Lowell’s enthusiasm and his interest and the fact that funds were allocated for this purpose that made this discovery possible, and I was very fortunate in being assigned to this as an amateur astronomer first to come here and have this wonderful opportunity to carry on this work.
I see. And then from then on, you went and got your degree.
Yes, then I got a scholarship at the University of Kansas and finished my academic work, and then I continued to search over other areas of sky later until I’d covered about three fourths of the sky. And then World War II came along and I became involved in war work, but the work was essentially finished, and I did come back and do a little more work on it afterwards to finish up what I thought was essential. So now the plates are being repeated for finding or discovering proper motion stars, stars which are near enough to the neighborhood of the sun and the Milky Way to show appreciable displacement, the interval now being 30 years instead of two or three nights. And so the plates that we used on the planet search (???) constituted the first epoch of the proper motion survey, and this has been very fruitful under the direction of Mr. Gitles(?), who’s carrying this work on, and his assistant; and the by—product work has been of immense value to stellar astronomers. But as one of the by-products of the planet search itself, there were hundreds and hundreds of asteroids showed up on the search, and many of them were new, around 700. And that also afforded the means to check the positions of asteroids so that they would not be lost as they are perturbed by Jupiter and so on, and so the by-products of the research were really quite important also.
I see. You spoke a little earlier, before we started recording, about two major (???) of this telescope: one was the finding of…
The searching for planets and finding of Pluto.
And then the other was the proper motion survey which was the greatest of its kind in history.
You want me to say something about how the observatory came to be established?
Yes, and some information about Lowell and also the observatory.
Okay. I might mention a little something about the Lowell Observatory, how it happened to be, its origin and so forth. Way back in the 1880s and so on, Percival Lowell was an ambassador I believe to Japan and finally to Korea, and he had a six-inch telescope. He was rather fascinated by the discoveries of the Italian observer, Scoparelli, in the canals of Mars. And so he started observing and became more and more interested, and finally he decided that he would set up an observatory of his own. He searched around different places in the world and finally decided upon Flagstaff to set up an observatory. This was in 1894. He had borrowed a telescope for two years, and then two years later he had his own — a 24-inch very fine refractor — for this purpose, and he was very fascinated with the planet Mars. He thought that Mars exhibited signs of having had a past civilization or something like that, and of course now we know that some of the ideas and facts are different than that now, but he was very much interested in the planets; and he also became interested in the possibility of another planet beyond the orbit of Neptune. And he understood quite a project employing several mathematicians and so on, and he directed the work — he was a good mathematician himself — studying the residuals of Uranus to get some possible clues as to where it might be in the sky, because the big problem of course is the large number of stars, millions and millions of them, and that’s why he did that. But he was also well known for Mars.
He wrote many very interesting books on Mars, and some of these I consider prize possessions, and he focused the attention of the world on the planet Mars. And of course as time went on, he added other telescopes to the observatory and the third major telescope of course was this 13—inch Lawrence telescope. Actually this telescope came after Percival Lowell’s death. He died in 1916, but his brother Lawrence Lowell, who had been president of Harvard for so many years, gave the funds to complete this more powerful wide-angle sky camera, which would be suitable for looking for planets. And so the Lowell Observatory today is known all over the world for the study of the planets and also for the discovery of Pluto. It would also extend other fields of work, like the proper motion stars today; spectra, photoelectric observations of variable stars and many other projects.
Just out of curiosity, funding — was this done through the government? Was it through private…?
My understanding was that much of the earlier work — in fact, I believe all of it — was by his own personal fortune. He belonged to the Boston Lowells, who had means to carry out these interesting projects, and he did this at his own expense for many decades. Now, of course, with the modern trend of things, some monies are now available for extending the work, enlarging on it, which didn’t exist back in the early days. So the Lowell Observatory was a private observatory, entirely financed, I understand, by the Lowell estate.
When you arrived here, Dr. Tombaugh, how many astronomers were employed at Lowell roughly?
Well, at the time I came and for a few years there were the three senior members of the Observatory — Dr. V.M. Slipher, who worked with spectra, and the director; and E.C. Slipher, his brother, who carried on the work of Mars and the planets, the direct photographs. And then Dr. Lampton(?) did a lot of work on the radiometric — the relation of the temperatures of the planets and photographing nebulae. And I was for a while, one year, the only junior member of the staff. And then later Mr. Giffis joined the staff and others from time to time, and so the staff grew. (pause in recording)
Okay, shall we start?
One of the other projects done in later years was to the search for possible rocks revolving around the earth as miniature moons. We have the examples of Saturn’s ring, which was made up of an enormous number of particles revolving around the plane of its equator, and although we know the earth didn’t have any such ring as that, we weren’t really sure whether there might be some rocks revolving around in the plane of the equator also. But because of our nearness to them, they would have a very large apparent motion which they would fail to register on routine astronomical photography. So I thought about this problem and decided to get a special mechanism to drive the cameras at different and rather high rates of speed so as to nearly match the apparent motion and this way conserve the light to a point or a very short dash instead of a long one and thereby reaching faint objects or rather small objects. We carried the work on here with some of this equipment for a few years, and in a way the failure to find any sub objects was a good sign, because it meant that the coast was reasonably clear. Assuming that someday there might be astronauts in space, would they be hit by these rocks if they were and so what do we know about this? And I think we essentially found the answer that the space was reasonably free of this and that the risk would be very low. And as it turned out, this sort of paved the way of us having some knowledge of the content of the space between the earth and the moon, and we know the astronauts went to the moon and landed and got back safely. They weren’t hit by rocks, and one of the answers we learned was out of this search for small and natural earth satellites, of which none were found.
I might add a few things about the early searches for the Planet X. In 1905 a search was initiated at the Lowell observatory by a 5-inch (???) lens, somewhat the same procedure of taking the plate, taking a para plate separated by a few nights, and this was done by overlaying one plate with the other and using a hand magnifier, a very tedious and perhaps more uncertain in thoroughness. And Percival Lowell did much of the examining of this himself. He was of course disappointed that the planet didn’t show up as he felt quite sure it existed up there. And then they attempted to try again in later years, and finally in 1914 they borrowed a 9-incfh wide-angle camera from the University of Pennsylvania for a couple of years, and as it actually turned up some of the (???) were on those but extremely weak. And by that time they had obtained a nice Blink microscope from Paris or from Germany which was much more thorough and much easier to examine these plates. They, of course, were disappointed that they didn’t find something, and Percival Lowell died in 1916. And so they decided maybe to try measuring it with a more powerful instrument, and this did not come about until after Percival Lowell’s death in 1929 as the 13-inch telescope was ready to go. So these are things you try for again and again. Many astronomers had given up hope of finding another planet beyond Neptune. They thought that many searches had been made and began to abandon the idea that any might exist. In fact, some were even wondering whether it was worthwhile to make the new attempt. But it did come about and with the larger telescope great refinements had to be made because of the sensitiveness of the instrument to refraction and driving rates and massive (?) plates under the Blink microscope and so on, and I happened to come in on that, and after much hard work came upon the images. And then many astronomers felt that the research should be continued over a larger area of the sky just to see if anything else would be picked up of that nature, and so I did; and for 13 more years I combed the skies without finding anything more, but we found a lot of by-product material.
So that I would be willing to say that I am very doubtful that any other planets exist beyond the orbit of Neptune other than Pluto that was within the reach of the instrument. This particular telescope had the capacity and had the capacity on these plates to pick up a planet like Neptune at seven times Neptune’s distance from the sun, and so there’s an enormous region of space there that I feel very certain that there are no major planets. And with such a big gap, one is doubtful if there are any other planets unless there are some little bodies beyond the reach of the instrument. So this was quite a long story, and the proper account of this I wrote as an article for the (???) Magazine in the March, 1960 issue. A more detailed and scientific account is found in Chapter 2 which I wrote for Kuiper’s Volume 3, The Planets and Satellites of the University of Chicago Encyclopedia of the Planets. The Blink microscope comparator (?) itself is a rather interesting gadget. When one has so many hundreds of thousands of images on a plate, one of these large plates, to examine, it’s a real problem how do you cover them thoroughly. This instrument has a place to put a plate and the other side one on another date. For planetary searches you want them only a few nights apart. If you want stellar searches, you want them several decades apart. This has a little prism arrangement or rather a mirror that goes out to each side of the plate — all these plates, of course, have to be precisely matched in exposure time and magnitude reach and so forth — and this device shows you first one plate, and then its shutter changes over (that is in the inside of it) and again it cuts one plate out and opens up the other until the other is in full exposure and then it’s cut off. Then it goes back again, and so that if the plates are well matched, you’re hardly aware you’re looking at two plates in rapid succession. But let anything change like a motion or shift of a body in that interval of time or a star changes brightness from brighter to less bright, it shows up immediately and conspicuously and permits one to do a thorough job and cover an enormous amount of star images. I would say that in addition to the telescope and the refined techniques that this required, which took a lot of testing, that the Blink microscope was an equally important part of this job because I cannot myself imagine how we would have accomplished such a search without a device like the blink microscope.