| Term | Definition |
|---|
| Planet | This word means "wanderers" in Greek |
| Solar system | Includes the sun, the planets and many smaller objects |
| Astronomical unit | The average distance between the sun and Earth, approximately 150,000,000 km |
| Telescope | Until this was created in the 17th century the universe was thought to have only eight bodies |
| Inner planets | Also known as terrestrial planets because their surfaces are dense and rocky |
| Outer planets | These planets are large and composed mostly of gases |
| Prograde rotation | the counterclockwise spin of a planet or moon as seen from above the planet's North pole |
| Retrograde rotation | The clockwise spin of a planet or moon as seen from above the planet's North pole |
| Magellan spacecraft | Mapped the surface of Venus by using radar waves between 1990 and 1992 |
| Water | This exists on earth because the temperature isn't too hot so it boils away or too cold so that it freezes |
| Ice | The only form of water found on Mars |
| Period of rotation | Amount of time it takes a planet to rotate on its axis |
| Period of revolution | Amount of time it takes a planet to orbit around another body |
| Olympus Mons | The largest mountain in the solar system--on Mars |
| Gas giants | The outer planets, other than Pluto |
| Saturn | Outer planet with the largest rings |
| Uranus | Planet that is tipped over on its side |
| Neptune | Planet that was discovered in 1846 |
| Organic compounds | What creates the many colors of Jupiter's atmosphere |
| Great Red Spot | A storm system on Jupiter that is more than 400 years old and 3 times the diameters of Earth |
| Satellites | Natural or artificial bodies that revolve around larger bodies such as planets |
| Luna | Another name for the Earth's moon |
| Phases | The different appearances of the moon due to its changing position |
| Eclipse | When the shadow of one celestial body falls on another this occurs |
| Solar eclipse | This happens when the moon comes between the Earth and the sun |
| Phobos and Deimos | The two moons of Mars |
| Galileo | Discovered the four largest moons of Jupiter--Ganymede, Callisto, Io and Europa |
| Titan | The largest moon or satellite of Saturn |
| Comet | A small body of ice, rock, and cosmic dust loosely packed together |
| Asteroid | A small, rocky object that oribts the sun, usually in a band between the orbits of mars and Jupiter |
| Asteroid belt | The region of the solar system that is between the orbits of Mars and Jupiter and in which most asteroids orbit |
| Meteroid | A relatively small, socky body that travels through space |
| Meteorite | A meteroid that reaches the Earth's surface without burning up completely |
| Meteor | A bright streak of light that results when a meteoroid burns up in the Earth's atmosphere |
| The Torino Scale | A system that allows scientists to rate the hazard level of an object moving toward Earth |
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