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Is it true that Pluto is not always the outermost planet in the solar system?

Yes. Pluto travels on a tilted orbit that crosses the plane shared by all other planetary orbits. (A plane is an imaginary surface that runs through all the points of an orbit.) Although Pluto's orbit is usually outside that of its closest neighbor, Neptune, at times it crosses over Neptune's orbit. For example, Pluto moved inside Neptune's orbit on January 23, 1979, making Neptune the farthest planet from the sun. Pluto will remain inside Neptune' s orbit until March 15, 1999.

Pluto was discovered in 1930 by American astronomer (a scientist specializing in the study of matter in outer space) Clyde Tombaugh (1906-). It is by far the smallest planet in the solar system, with a diameter of 1,457 miles (2,344 kilometers). Pluto is thought to be composed of rock and water ice. There are bright areas on the planet's...

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1. Earth And Sun
   1. How hot is the sun?
   2. When will the sun die?
   3. When do solar eclipses happen?
   4. What is the safest way to view a solar eclipse?
   5. When will the next total solar eclipse be visible in the United States?
   6. What is the distance from Earth to the sun?
   7. Is it true that during the Northern Hemisphere winter, the Earth is closer to the sun than it is during the Northern Hemisphere summer?
   8. What are the diameter and circumference of the moon?
   9. What is the circumference of the Earth?
   10. What is solar wind?
   11. What are auroras and how often do they appear?
2. Planets And Moons
   1. How old is the solar system?
   2. How far are the planets from the sun?
   3. What are the diameters of the planets?
   4. What are the colors of the planets?
   5. Which planets have rings?
   6. How many moons does each planet have?
   7. How long do the planets take to go around the sun?
   8. Is a day the same on all planets?
   9. How can an observer distinguish planets from stars?
   10. Is it true that Pluto is not always the outermost planet in the solar system?
   11. What is Planet X?
   12. Is there life on Mars?
   13. How far is the moon from the Earth?
   14. What are the phases of the moon?
   15. Is the moon really blue during a blue moon?
   16. Why does the moon always keep the same face toward the Earth?
   17. Why do Iunar eclipses happen?
3. Meteors, Asteroids, And Comets
   1. How does a meteorite differ from a meteor?
   2. When do meteor showers occur?
   3. What are the largest meteorites that have been found in the world?
   4. Where are asteroids found?
   5. What was the Tunguska Event?
   6. Where do comets originate?
   7. What is Comet Hale-Bopp?
   8. When will HaIley's comet return?
4. Stars, Galaxies, And The Universe
   1. Which star is closest to Earth?
   2. How are stars born and how do they die?
   3. What is a black hole?
   4. What does the color of a star indicate?
   5. What is the Big Dipper?
   6. How many constellations are there and how were they named?
   7. Where is the North Star?
   8. What is the Milky Way?
   9. How old is the universe?
   10. Which galaxy is closest to us?
   11. What was the "Big Bang''?
   12. What is a cosmic string?
   13. What is a pulsar?
   14. What are quasars?
5. Space Travelers And Spacecraft
   1. Who was the first man in space?
   2. Which astronauts have walked on the moon?
   3. Who were the first man and woman to walk in space?
   4. Who was the first woman in space?
   5. Who was the first African American in space?
   6. When was the first animal sent into orbit?
   7. What are some of the accomplishments of female astronauts?
   8. Who was the "father" of the Soviet space program?
   9. What was the worst disaster in the U.S. space program and what caused it?
   10. How many fatalities have occurred during space-related missions?
   11. When was the first United States satellite launched?
   12. What did NASA mean by Voyager 2 taking a "grand tour" of the planets?
   13. What is the message attached to each Voyager spacecraft?
   14. What is the mission of the Galileo spacecraft?
6. Astronomers And Skywatching
   1. Who is considered the greatest ancient Greek astronomer?
   2. For whom is the Hubble Space Telescope named?
   3. How did a total solar eclipse confirm Einstein's theory of general relativity?
   4. Who invented the telescope?
   5. Who is Stephen Hawking?
   6. Is anyone looking for extraterrestrial life?
   7. What is a "close encounter of the third kind"?