Question

read the passage.
the device that captured pluto
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 lowells 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, or central, idea of the passage?
although percival lowell posited the existence of pluto, clyde tombaugh was the astronomer who ultimately found it.
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.

Explanation:

The passage details how Percival Lowell theorized a planet beyond Neptune, and how Clyde Tombaugh used a blink - comparator to search for it. The key point is that the blink - comparator was the tool that led to Tombaugh's discovery of Pluto.

Answer:

A powerful dual microscope called the blink comparator enabled Clyde Tombaugh to discover Pluto.