| A | B |
|---|
| astronomical unit (AU) | the average distance between the Earth and the sun; approximately 150 million km. |
| terrestrial planets | one of the highly dense planets nearest to the sun ;Mercury, Venus, Mars, and Earth |
| prograde rotation | the counterclockwise spin of a planet or moon as seen from above the planet's North Pole; rotation in the same direction as the sun's rotation |
| retrograde rotation | the clockwise spin of a planet or moon as seen from above the planet's North Pole |
| Magellan | spacecraft that mapped the surface of Venus by using radar waves |
| gas giants | a planet that has a deep, massive atmosphere, such as Jupiter, Saturn, Uranus, or Neptune |
| satellite | a natural or artificial body that revolves around a planet |
| phases | the change in the sunlit area of one celestial body as seen from another celestial body |
| eclipse | an event in which the shadow of one celestial body falls on another |
| comet | a small body of ice, rock, and cosmic dust that follows an elliptical orbit around the sun and that gives off gas and dust in the form of a tail as it passes close to the sun |
| asteroids | a small, rocky object that orbits the sun, usually in a band between the orbits or 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 |
| meteoroid | a relativley small, rocky body that travels through space |
| meteorite | a meteoroid 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 |
