| A | B |
|---|
| universe | everything that exists |
| galaxy | a large group of stars, gas, and dust held together by gravity |
| solar system | the Sun, the planets, and many smaller objects that travel around the Sun |
| star | a hot, glowing sphere of gas that produces energy by fusion |
| planet | a body of matter that orbits a star |
| moon | a body of matter that orbits a planet |
| comet | a large chunk of ice, frozen gases, and rock fragments that moves through space |
| asteroid | a large chunk of rock orbiting the Sun |
| meteoroid | small piece of rock moving through space |
| meteor | a small piece of rock falling through the atmosphere |
| meteorite | a meteor that strikes Earth's surface |
| satellite | object that orbits a planet |
| revolve | to move around an object; orbit |
| rotate | to spin on an axis |
| Astronomical unit | average distance between Earth and the Sun |
| light year | distance light travels in a year |
| continental drift | hypothesis that Earth started with one supercontinent which broke into smaller landmasses and moved apart |
| plate tectonics | theory explaining the movement of the continents |
| plate | one section of the Earth's crust and rigid mantle |
| convection cell | current of heated molten rock causing a plate to move |
