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.
What is the main idea of the passage?

The blink comparator enabled Clyde Tombaugh to compare photographs taken at different times. 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.
What is the main idea of the passage?

The blink comparator enabled Clyde Tombaugh to compare photographs taken at different times.