Jump to: Before Tombaugh, Tombaugh Arrives, Searching, Naming Planet X

BEFORE TOMBAUGH:
       The search for "Planet X" first began in 1905. Using a 5-inch telescope, Percival Lowell took pictures of the sky for two years. To study them, he laid one plate over the other and used a magnify glass to look for any shift in the stars. He did not have much luck. Today, astronomers have realized that even the best technology would not have helped in the search for Pluto. Pluto was outside Lowell's search area and would have been too faint to be recorded. To find Planet X was Percival Lowell's dream. He studied the pattern of Uranus's orbit, and predicted the position of Planet X in the Libra sky. Again in 1911, a search was done- this time, with a 40-inch reflector. This search also ended unsuccessfully.
        In 1914, Lowell borrowed a 9-inch camera from Swarthmore College. Almost 1,000 pictures were taken of the sky. Technology had advanced enough that Lowell now used a Carl Zeiss Blink-Comparator to scan the plates. However, because the pictures were taken on March 19 and April 17, 1915 (possibly the worse time to discover Pluto because of its orbit), Pluto was not in any of the pictures. Had the astronomers waited until December, they might have had more luck. "The 'quick find' attitude seemed to dominate in the earlier searches, resulting in improper procedures, and lack of thorough checking of planet suspects. They were looking for a planet of the 13th magnitude (10 times brighter than Pluto)."
       In 1915, Lowell revised his prediction of Planet X's location. The new region, eastern Taurus, was rich in stars. This slowed down the search. Unfortunately, on November 16, 1916, Lowell died of a stroke at the age of 61. The search for Planet X stopped for the next 13 years.
       Lowell Observatory underwent a series of ownership changes after the death of Percival Lowell. Finally, in 1927, Roger Lowell Putnam, Percival Lowell's nephew, became the Observatory's sole trustee. Determined to finish this uncle's dream, he convinced another uncle, Dr. A. Lawrence Lowell (President of Harvard University) to provide $10,000 to improve the Observatory's equipment.
       V.M. Slipher was put in charge of designing a new telescope. The new telescope had a 3-element Cooke type astrograph objective lens. This was the telescope Clyde W. Tombaugh would be hired to use. After sending the Lowell Observatory several drawings of Jupiter and Mars that he made with his 9-inch, f9, reflector, Tombaugh was hired by Slipher in 1928.

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TOMBAUGH ARRIVES:
       Clyde Tombaugh brought with him many new ideas and ways to search for Pluto. The first thing he did was to fix the problem the Observatory was having with the plates of pictures. These plates, 14 inch by 17 inch, needed wooden frames to bend them to the exact Petzval curvature of the field before being placed in the telescope. Some of these plates would break in half during the hour-long exposure. While the other astronomers were willing to accept a few broken plates, Tombaugh couldn't. The schedule was already too busy without having to repeat plates. He solved this problem by changing the order the screws were placed on the wooden frames. The strain on the plates diminished and there were no more wasted plates.
        Another problem Tombaugh solved was the "pulsing" at certain hour angles. To solve this problem, the observer would be required to manipulate the east-west button on the control box of the drive mechanism every once in awhile. This could soon wear out the skywatcher. Tombaugh solved this problem by moving the counterweights on the Declination axis. This created more torque on the driving clock and would keep the worm screw in constant contact with the worm wheel and there was no more "pulsing."
       Double images were also causing obstacles for the astronomers. A sudden shift on the Declination axis would displace the guide star off the crosshairs of the telescope. The observer could manually move the telescope so that the guide star would now be back on the crosshairs, but when the pictures were developed, the images were exact doubles. This problem disturbed Tombaugh for many nights until he noticed it always occurred at 0h42m west of the meridian. To avoid double images, Tombaugh discovered that all one needed to do was to swing the telescope westward before the exposure was to go through that critical angle.
       Finally, the last problem Tombaugh had was one that he had little control over. (Or did he?) The large camera was sensitive to the "steadiness of seeing." The quality and magnitude of the image needed to be very similar. An image with a "2" seeing could not be compared, using the blink comparator, to an image taken with a "4" seeing. Many things factored into the quality and magnitude of the image, such as the weather. How could Tombaugh control the weather each night? He couldn't. So, he had to schedule taking plates on nights with about the same quality of seeing. And if there were a slight haze, he would prolong the time of exposure to maintain the magnitude limit.

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SEARCHING:
       The actual search for Pluto began in April 1929. Lowell's prediction of the planet had once again been revised. This time, the planet was predicted in Gemini. Pictures were taken of Gemini and were compared using the blink comparator as soon as possible. All the plates were completed in two weeks, way too fast to be able to carefully scan approximately 700,000 stars. In the rush to compare the plates, the astronomers missed the images of Pluto, which had been captured on the Delta Geminorum plates. More pictures were taken. This time, they included the whole sky, not just Gemini.
       By June of 1929, one hundred plates had been taken, but only a few had been blinked. It was a very tedious task. The star density of the Milky Way, particularly Western Gemini where the search was focused, was around 400,000 stars per plate. And how was an astronomer supposed to tell the difference between an asteroid and Planet X?
       In September 1929, Tombaugh began taking pictures of the Pisces sky. There were several false planet suspects on nearly every plate. In order to check these suspects more carefully, Tombaugh needed to take a third picture. Taking three plates had advantages as well as disadvantages. Although it would waste some time to take them, having a third plate allowed the astronomers to select the best-matched pair to blink.
       In January 1930, Tombaugh photographed the Gemini sky. On January 21, he started the Delta Geminorum region. But as soon as he started, a strong wind began to pick up. The guide star started moving around, swelling in angular diameter and then fading away. After an hour, Tombaugh quit. It was the worst seeing he had ever experience.
       But the pictures showed Pluto's image in the right place! On February 18, using plates from January 23 and January 29, Tombaugh detected a slight shift that turned out to be Pluto. The image from January 21 was consistent with these plates. More plates verified this discovery. Planet X had been discovered.

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NAMING PLANET X:
       With the discovery of Planet X, Percival Lowell's dream had been accomplished. But what should they name Planet X? Mrs. Lowell wanted the planet to be named Lowell. Other sources say Mrs. Lowell wanted the planet to be named Zeus. But in any case, she later changed her mind and wanted Constance. Suggestions also included Atlas, Zymal, Artemis, Perseus, Vulvan, Tantalus, Idana, and Cronus. The New York Times was believed to have suggested Minerva. Reporters suggested Osiris, Bacchus, Apollo, and Erebus. The astronomers at Lowell Observatory favored Cronus, Minerva, and Pluto. An 11-year-old schoolgirl in Oxford, England named Venetia Burney first proposed the name Pluto. (But hey, who actually listens to 11-year-old girls?) A few months after the discovery of Planet X, it was officially named Pluto, after the god of the underworld. Perhaps it was because the planet is in perpetual darkness as a result of its distance from the sun or because "PL" was the initials of Percival Lowell.

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