| A | B |
|---|
| heliocentric | the sun is the center of the universe |
| geocentric | the earth is the center of the universe |
| planet | a massive object orbiting a star like the sun |
| solar system | the sun and all objects that are bound by gravitational force to the sun |
| gravitational force | the force of attraction between all objects |
| law of universal gravitation | states that the strength of the gravitational force depends on the mass of the objects and the distance between them |
| orbit | regular, repeating path that an object in space follows around another object |
| satellite | an object in orbit around another object |
| axis | the imaginary line that passes through the center of a planet from pole to pole |
| astronomical unit | equal to 150 million km, or the distance from Earth to the Sun |
| terrestrial planets | made mostly from rock and metal; include Mercury, Venus, Earth and Mars |
| gas giants | made mostly of hydrogen and helium; include Jupiter, Saturn, Uranus and Neptune |
| asteroid | an object that orbits the Sun, but is too small to be considered a planet |
| comet | an object in space made mostly of ice and dust |
| meteor | a chunk of burning rock traveling through Earth's atmosphere |
| meteorite | a meteor that passes through Earth's atmosphere and strikes the surface |
