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    Clyde Tombaugh Signed Typescript Regarding His Discovery of the Planet Pluto. An 8.5" x 11" single page document as he would send out to interested parties. Signed at close "Clyde W Tombaugh/ 16 October 1996". Quite interesting to read. Full transcript online. Very fine.

    More Information: Since I receive nearly 100 letters every month, this form letter will answer most common questions. Many people think that I found Pluto by looking in the eyepiece of a telescope at night.

    Not so. Pluto looks like any faint star among some 15 million stars as bright or brighter than Pluto. Then, how does a searcher recognize it as a planet?

    The only clue is its very slow motion against the rich star background. Because of Pluto's great distance, one would have to watch for 2 hours, before its movement was perceptible. But, there are about 1,000 stars in each square degree that would have to be watched! It is obvious than that a faint planet like Pluto, cannot be found by visual looking.

    One must freeze every object on a one hour exposure photographic plate. Then re-photograph the same region 2 to 6 nights later. The pair of plates is then scanned by rapid alternating views thru the eyepiece of the Blink- Microscope-Comparator. One generally finds several false planet suspects. So one needs a third plate taken within a week to be checked for confirmation! If the image on the third plate is consistent with the shift noted on the pair, then you have a planet!

    Generally about one dozen asteroids are recorded on each plate. They move also. How do you distinguish an asteroid from a much more distant planet? In two wide sectors of the sky, asteroids appear to slow down and imitate the very slow motion of a more distant planet, due to the orbital geometry. One can avoid this confusing situation by taking the plates when the region is in opposition (that is 180 degrees from the sun). Since this region moves eastward 30 degrees every month, one has only one month of "observing window". At this particular time the Earth is moving crosswise to the direction of the planet. This provides a "running parallax" enabling one to determine the distance of the planet, that the object is truly trans-Neptunian. In this position the asteroids shift about 7 millimeters per day interval, whereas, Pluto shifted only 2 millimeter per day.

    In scanning each pair of plates, 50,000 to 900,000 star images passed in review in the eyepiece of the comparator! A long and tedious job, indeed. One must take frequent work breaks in order to maintain the intense mental attention, otherwise one might fail to note the shifting image of a planet.

    The regions in the Zodiac Belt were searched first, which chanced to contain Pluto. This belt is 12 degrees wide and the sun, moon and planets are always found in it. When Pluto's orbit was computed, it had an inclination of 17 degrees! Therefore, to make a complete search for planets, I was encouraged to search over wider areas of the sky. But, no more planets showed up, so one may presume that more planets in the solar system do not exist. Pluto is a very dim object in the sky, equivalent to one candle as seen from a distance of 300 miles! My plate limit recorded stars 5 times fainter than Pluto.

    There were by-product discoveries: six new star clusters, two comets, several hundred asteroids and variable stars, clusters of galaxies, and one super-cluster of galaxies! A total of 29,500 galaxies I counted on my plates! I had a tremendous tour of the universe. A total of 338 pairs of 14x17 inch plates were carefully scanned in search of planets, the most thorough search in all astronomical history. 90 million star images passed in review during 7,000 hours of plate scanning, over a period of 14 years.

    It is customary to name planets after the old Greek and Roman gods of mythology. The asteroids got most of the available names. A few not used were Zeus, Cronus and Pluto. Since Pluto was the god of the dimly-lit underworld, Hades, the Lowell staff agreed on Pluto. Each member had one vote. The first outside suggestion for Pluto came from a 11 year old girl in England, Venetia Bierney.

    The fissionable element Plutonium was named after the planet, like Neptunium after Neptune.

    Over the years, many articles about Pluto have been published in Sky & Telescope and also other magazines. See your library (city, school, or university) for my book Out of Darkness: The Planet Pluto, published in 1980, but now out of print. The publisher badly under estimated the market. For one of the more recent articles see my paper in the April 1991 issue of Sky &t Telescope.

    Also, see your Encyclopedia under the entries of: Pluto and Tombaugh.

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