Pluto (Encyclopedia of Science)
Pluto, the ninth and farthest planet from the Sun, is one of the least well understood objects in the solar system. It is the smallest of the major planets and has a most unusual orbit. Pluto is only 1,428 miles (2,300 kilometers) in diameter. Since the planet is 3.66 billion miles (5.89 billion kilometers) away from the Sun, it takes almost 250 years for it to complete one revolution around the Sun. However, it takes Pluto only 6.39 Earth days to complete one rotation about its own axis.
In Greek mythology, Pluto is the god of the underworld. The planet was given its name for several reason. First, due to its great distance from the Sun, Pluto is almost always dark. The sunlight it receives is about equal in intensity to moonlight on Earth. Second, Pluto is the mythological brother of Jupiter and Neptune. And finally, the planet's name begins with "PL," the initials of Percival Lowell (1855916), the American astronomer who spent the final years of his life searching for the elusive planet.
The search for Pluto
Shortly after the discovery of Neptune in 1846, astronomers began looking for an even more distant planet. They believed some celestial body existed at the outer reaches of the solar system that caused disturbances in the orbit of Uranus. The gravitational field of Neptune accounted for some of its neighbor's orbital irregularities, but...
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