The Surprises of Pluto
Ever since Pluto was discovered, it has been described as the planet farthest from the sun.Its orbit makes a huge sweep that it takes Pluto 248 years to traverse, instead of the single year in which Earth completes one turn about its own small orbit.
Pluto's orbit, however, is distinctly elliptical, with the sun well to one side of the center.When Pluto is at the end of its orbit that is farthest from the sun, it is 4.6 billion miles away, and it is then 1.7 times as far away from the sun as is Neptune, the next farthest planet.
Every 248 years, however, Pluto moves around to that section of its orbit where it is closest to the sun, and then it is only 2.7 billion miles away. Surprisingly, it is at that time actually a trifle closer to the sun than Neptune is.For twenty years it skims along that portion of its orbit, remaining closer than Neptune.Then it passes beyond Neptune's orbit again and begins its long trek outward to the vast distances beyond.
In January 1979, Pluto passed inside Neptune's orbit.Pluto is therefore not the farthest planet from the sun; Neptune is, and will be until 1999, when Pluto will resume its position as farthest planet and won't pass inside Neptune's orbit again till 2227.
Pluto was first discovered because the outer planets, Uranus and Neptune, don't move quite exactly as the law of gravitation predicts.The difference is minute, but some astronomers wondered if there might be another planet beyond Neptune whose gravitational pull wasn't being allowed for.If the planet's pull were taken into account, that might explain the discrepancy in the motions of Uranus and Neptune.
About 1900, the astronomer Percival Lowell calculated where the distant