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Children's LiteratureThe word planet comes from the Greek word for wanderer. That is because the Greeks noticed that there were some "stars" in the sky that move around in the star patterns, or constellations. These objects were not stars at all, but planets. Six planets were visible with the naked eye and discovered by ancient astronomers before the invention of the telescope. In the last 200 years three more planets have been discovered. Each of the nine planets circles around the sun. A year is the amount of time it takes for a planet to move around the sun. A day is the amount of time it takes for a planet to revolve on its axis. Every planet's day and year is unique to that planet. Did you know that Venus's day is longer than its year? Saturn's day is only 11 hours long, but its year is 30 Earth years long. Saturn, Jupiter, Uranus, and Neptune are gas giants. They are made up mostly of gases. They all rotate faster than the Earth despite being much, much larger. Nevertheless, their years are much, much longer. Neptune's year is 164 Earth years. The planets are fun to study and most kids find them fascinating. 2005 (orig. 1993), Holiday House, Ages 5 to 8.