Sunday, August 24, 2008

It's better than Oz!

If you say "Kansas" to just about any American, the first thing that probably pops into his or her head is The Wizard Of Oz. Or corn. Or wheat. Or flat and boring.

But Kansas has a much more interesting claim to fame. Pluto!

Most planets don't have an official discoverer. They are easily visible and while they weren't correctly identified as planets until long after they were first seen, there is no one person listed in the history books as the person who discovered them. The orbits of Uranus (William Herschel, 1781) and Neptune (Johann Gottfried Galle and Louis d'Arrest, 1846) led astronomers to believe that there was still yet another planet out there to be found, and Dr. V. M. Slipher, director of the Lowell Observatory, hired Clyde Tombaugh to find it.

Tombaugh was only 22 years old and had already built a series of telescopes, each more powerful than the previous, in order to make detailed drawings of Mars and Jupiter. He sent them to the Lowell Observatory and was hired soon after, in January 1929. How he found it is an amazing display of patience and attention to detail:

Each night, when the weather was clear and the Moon was not shining, Tombaugh placed a photographic plate, 14 by 17 inches in size, at the focal point of the 13 inch telescope. The plate had to be held firmly in place by screws, so that it would not shift during its three-hour exposure. It also had to be held at an extremely precise curvature, to an accuracy within half a millimeter, so that the entire large plate would be in focus. Tombaugh would then point the telescope at a precise region of the sky, and make sure that the telescope would rotate slowly throughout its exposure time to compensate for the Earth’s own rotation. After three hours he would remove the plate, replace it with a new one, and point the telescope to a new sliver of sky, adjacent to the previous one. Every few days he would retrace his steps, and record images of the same region he had taken previously. As a result, he possessed at least two plates of each observed section of the sky, taken several days apart. The entire procedure was conducted nearly every night in an unheated dome, often in sub-zero temperatures.

The sequence in which Tombaugh chose to survey the sky was not random. Always he insisted on observing only those regions that were in “opposition” – i.e. directly opposite from the Sun. Since the Earth takes a full year to complete its orbit around the Sun, it would therefore take a year to complete a full survey of a band of the sky. The reason for this choice was that Tombaugh was looking for an object, Planet X, that would exhibit “retrograde motion” – the apparent motion of a planet or asteroid “backwards,” or east to west instead of west to east. For the outer planets, those outside the Earth’s orbit, this occurs when the Earth races by them, leaving them behind and creating the illusion that they are moving backwards. This only happens when the Earth is between the retrograding planet and the Sun, i.e. when from the perspective of an observer on Earth the planet is in opposition. Detecting an object in retrograde motion immediately indicates to an observer that the object is relatively close-by, probably within the Solar System. The stars are so distant that they hardly show any shift in position (or “parallax”) at all due to the Earth’s annual orbit, and their minute shift can only be detected through careful measurement by specialized instruments.


Over time he accumulated an impressive set of photographic plates, two each for each region of the sky. But as the plates multiplied, still no work was being done on scanning them carefully in search of an elusive planet. That would involve mounting the plates two at a time on the old blink comparator and going over them, tiny sliver by tiny sliver, with the centrally mounted microscope. Tombaugh could not help but wonder: “who would blink these plates?”

The answer came in June: Tombaugh, who had been hired specifically to conduct the telescopic survey, would now be responsible for the blinking as well. The Lowell Observatory’s entire search for Planet X was now in the hands of a 23 year old with a high school education, who six months before was working on his family’s farm. Once he got over his initial surprise, Tombaugh began filling his days and nights not occupied with observing with hours upon hours of “blinking.”

Blinking was a tough job that was at the same time extremely tedious and requiring exquisite concentration. Tombaugh set about it with his usual systematic approach. After half an hour of blinking he would take a short break, and after another half hour he would walk away from the comparator for at least 15 minutes. “I knew I dare not overdo it or my attention would lapse and I could miss something” he recalled years later. “This haunted me all the time.”

As the months went by Tombaugh became an expert blinker, working at the comparator for nine hours a day when he was not occupied with the observations themselves. He found numerous objects moving in retrograde motion, but judging by their rate of motion they were too close to the Earth to be Planet X candidates, and Tombaugh concluded that they were asteroids. Inch by inch, star by star, Tombaugh would scan the plates. By January 1930, by his own estimate, he had already scanned a million and a half(!) stars, and still - no result. (Source)

It's been estimated that Tombaugh spent 7000 hours hunched over the blink comparator until February 18th, 1930, searching those plates a star at a time. Do you see it? Can YOU see the one tiny speck of light that changed position?

There it is.


(There's a fun side story about how Pluto was named, if you're interested.)

In 1978, Pluto's first moon was found by James Christy because of images like this. The blob in the middle isn't quite spherical. The bulge wobbled around the planet in subsequent images. The moon was called Charon.

It wasn't until very recently, May 2005, that the moon count for Pluto changed when Hubble imaged Hydra and Nix. It is thought by some that Pluto may have more moons, or even rings, still yet to be found.

That will change soon, however, as the New Horizons mission is on its way to Pluto. It was launched in January 2006 and will arrive in July 2015.

Regardless of how Pluto is classified, Kansans in the know will always be able to say we found it first. And trust me, traveling from here and hearing joke after joke about Oz, we're happy to have something else to brag about.

Read more about Pluto with Bill's Plutopia blog carnival.


  1. Great post, very informative. Pluto is still a planet in spite of a vote of four percent of the IAU. Read more about the pro-Pluto view on my blog at

  2. For the record, I both applaud the ‘debate’ as a teachable moment of the Nature of Science and shudder at the mere idea that Pluto will be forever cast out of planetdom.

    Pluto will always be a planet to me. Regardless of what the ‘powers-that-be’ declare.

    As a bit of personal history, I took a planetary geology class many years back. The instructor divvied up the planets and other notable solar system objects and assigned a 45 minute presentation on each one. A few, Mars in particular, were team presentations. Most of the rest were solo projects.

    The students drew numbers out of a hat. I ended up with one of the last numbers and when my choice came, Pluto was one of the few remaining. I was thrilled that no one had chosen the "poor little thing" and snapped it up without hesitation.

    Now, this was in 2003. We know very little about Pluto in 2008 and knew a lot less in 2003. Nix and Hydra hadn't even been found yet. How was I supposed to fill up 45 minutes on the little we knew about Pluto?

    I loved it. I invited the entire department, faculty especially, to my class presentation. In the end, I had most of the audience holding various sized circles representing the other planets, figuring out which they held, putting themselves in the correct order from the sun, and then going out into the hallway to place themselves at their estimated distance from the sun (with a changed scale, obviously - the sun was a yellow push-pin stuck into the floor). The piece de resistance? The person holding Pluto (and being very careful not to drop the 1/4 inch circle) ended up down the flight of stairs.

    "Why," she asked, "am I down the stairs?" The rest thought I'd mismeasured. "Because Pluto is not on the ecliptic plane," I replied. I tried not to giggle, but it really was amusing seeing my department head standing in the middle of the hallway, holding my Solar System Measuring Device (toilet paper) in one hand, huge Jupiter representation in the other, enjoying herself immensely.

    Anyway, I had a blast. I established myself as the presentation to beat. No one else even came close. And since that day, I've been an ardent supporter in the "Pluto will always be a planet to me" club. Just ask my friends and classmates. They know how to push my planetary buttons.

  3. I love your story! I wish you had come to the Great Planet Debate in Laurel, MD, earlier this month! There were astronomers, teachers, writers, planetarium directors, members of the general public, etc. there, and we had three wonderful days of discussions on this issue. I believe people like you and me should have a voice in these discussions, and this conference gave us just that. The IAU has been doing the exact opposite, not only ignoring public sentiment but also excluding its own members who could not be in a particular room on a particular day and other planetary scientists who are not IAU members. That is in addition to ignoring the recommendations of their own committee and violating their own bylaws at the General Assembly by putting forward a definition that had not been vetted by a committee, as its procedure requires.

    My friends push my planetary buttons too, but it's all in fun. And I genuinely believe Pluto will not "be forever cast out of planetdom." There are too many people, including planetary scientists, who object to this demotion and refuse to use it. When New Horizons gets to Pluto in 2015, Pluto's planetary status will likely be confirmed once and for all. And there may just be a new set of "powers that be" led by some of the organizers of the Great Planet Debate! It's not over!