| Word | Definition |
| astronomical unit (AU) | the average distance between the Earth and the sun, or approximately 150,000,000 km |
| terrestrial planets | the small, dense, rocky planets of the inner solar system |
| prograde rotation | the counter-clockwise 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 |
| gas gaints | the large, gaseous planets of the outer solar system |
| satellite | a natural or artifical body that revolves around a planet |
| phases | the different appearance of the moon due to varying amounts of sunlight on the side of the moon that faces the Earth |
| eclipse | an event in which the shadow of one celestial body falls on another |
| comet | a small body of ice, rock, and cosmic dust loosely packed together that gives off gas and dust in the form of a tail as it passes close to the sun |
| asteroid | a small, rocky body that revolves around the sun |
| asteroid belt | the region of the solar system most asteroids occupy |
| meteoroid | a very small, rocky body that revolves around the sun |
| meterorite | a meteoroid that reaches the Earth's surface without burning up completely |
| meteor | a streak of light caused when a meteoroid or comet dust burns up int he Earth's atmosphere before it reaches the ground |