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Clyde Tombaugh sat down at an apparatus called a blink comparator, where he had spent hundreds of hours looking at photographic images of stars that appeared as 1 tiny little white specks on the black photographic plates. It was Thursday, February 13, 1930, and Tombaugh had been working at Lowell Observatory for about 2 thirteen months, he was looking for a planet that Percival Lowell had predicted would be at the far boundary of the solar system.3
Tombaugh picked up the next set of photographic plates; weeks earlier, he had taken the photographs by pointing a telescope at a particular spot in the night sky, days apart but at the same time. If Lowell was right, one of the white specks would be in two different positions on the two plates. Tombaugh 4 loaded the images side by side in the blink comparator and looked through the eyepiece to compare the enlarged images.
Like Lowell, Tombaugh believed there was an additional planet in our solar system, farther away than Neptune. Neptune's orbit did not exactly match the one predicted by calculations, and the presence of another planet could have caused that difference.5
Tombaugh moved a mirror in the viewer. It allowed him to look at a small area in the image on the left. When he moved the mirror again, he saw the image on the right in exactly the same place; the image did not change as he flipped back and forth, so he moved the images slightly and began looking at the next area, slowly working his way around the plates.
Tombaugh knew he could 6 check between a planet in our solar system and the stars; the planet, being closer, would change location in the sky relative to the stars. 7 Although, the farther the planet is from Earth, the more slowly that change happens, making it difficult to observe.
 Tombaugh continued to work his way, inch by inch, over the images, flipping back and forth with 8 the mirror; he saw no difference between the two images at each location, indicating he had nothing but far-away stars in the images.  Once more, he moved the images and flipped the mirror back and forth while looking through the eyepiece.  This time, as he blinked from one image to another, a speck seemed to move.  He knew immediately that it was significant; checking more images convinced him that he had found a planet, which was eventually named Pluto.  Pluto was named for the Roman god of the underworld, since it was cold and far from the sun.9
Since that time, scientists 10 has determined that Pluto is smaller than it initially appeared and is actually a very large asteroid, rather than a planet. 11 Pluto is, however, still a part of our solar system, located by calculations, careful observations, and a lot of patience.
3. Which sentence would most effectively conclude the paragraph?
5. Which sentence, if added at the end of the paragraph, would provide support for the central idea of the paragraph?
9. Which sentence in the paragraph is least important to the main topic of the paragraph?
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