In the early 1900s, astronomer Percival Lowell theorized the existence of a planet beyond Neptune; however, he was never able to find "Planet X." After Lowell's death, Clyde
Tombaugh took up the search, using a telescopic camera to photograph areas where he
thought Planet X might be found. Tombaugh used an amazing tool called a blink
comparator to analyze these photos. Equipped with two powerful microscopes, the device
allowed the user to examine two different photographic images taken at different times,
using a dial to switch rapidly between them. Tombaugh knew that planets, unlike stars,
move in orbits--so he looked for minute differences that could signify a moving body. In
February 1930, Tombaugh found two images, taken six days apart, that were very
slightly different; an object had changed locations. Thanks to the blink comparator, he
had found the elusive Planet X, now known as the dwarf planet Pluto.
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What is the main idea of the passage?
-The blink comparator enabled Clyde Tombaugh to compare photographs taken
at different times.
-A powerful dual microscope called the blink comparator enabled Clyde
Tombaugh to discover Pluto.