Be honest.  How many of you reading this post were upset when Pluto was “downgraded” to a dwarf planet?  I count myself in that group.   

When I was a child, I learned there were nine planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto.  I remember hearing about the vote on a new definition of a planet by the International Astronomical Union (IAU) in 2006.  This vote ended up re-classifying Pluto, probably correctly, as a dwarf planet, meaning there are only eight planets in our solar system.  There was much gnashing of teeth over the decision.  Interestingly, This Week in Science History marks an anniversary in the discovery of the dwarf planet Pluto.  

Percival Lowell caught glimpses of the existence of Pluto in 1905.  He observed deviations in the orbits of Neptune and Uranus which he attributed to the gravity of a ninth planet, called Planet X, not yet observed.  He predicted its location, but was unable to find it.  Along came 24-year old farm boy, Clyde Tombaugh.  Tombaugh lived on a farm in Kansas and used a small telescope to observe the heavens.  He made detailed drawings of the surfaces of Jupiter and Mars and used these to obtain a job at the Lowell Observatory in Arizona.  When the moon was dark and the weather was clear, Tombaugh would make 14 x 17 inch photographs of the night sky.  The precision on his measurements had to be excellent.  He would have to account for the movement and curvature of the earth, with accuracy to a half a millimeter, over a three hour period.  Several days later, he would have to precisely re-create the same photograph.  The result is the set of pictures attached to this post.  

On January 23, 1930, Tombaugh photographed the night sky and re-took the same section on January 29, 1930.  When he reviewed the photographs on February 18, 1930, he noticed a small dot which moved between the two photographs.  He had seen small dots moving before (these were asteroids), but this one was different.  He had discovered Planet X!  An 11 year-old girl, Venetia Burney of Oxford, England, suggested the name of the Roman god of the underworld, Pluto, for the planet.  The IAU would recognize the ninth planet later that same year until the discovery of other (and larger) objects in the Kuiper belt would reveal the definition of a planet to be insufficient causing the re-classification. 

What amazes me about all of this is the patience and rigor with which Tombaugh worked.  In order to make the measurements, he had to be extremely precise.  When looking at the photographs, he had to be patient and diligent because he was looking for a speck moving in a field of dots.  I went to Lowell Observatory in 2006 and had a chance to see reproductions of the photos above.  I can tell you that it was difficult for my untrained eye to see the difference, even with the arrow to point it out.  Simply amazing! 

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