| A | B |
|---|
| asteroids | rocky bodies in orbit between Mars and Jupiter |
| retrograde motion | the apparent backward motion of planets through the stars |
| comets | icy, rocky objects originating in a region outside the planets; have highly eccentric orbits |
| geocentric | earth-centered view of the solar system |
| aphelion | the farthest approach of a satellite to its primary; where a satellite moves slowest in its orbit |
| perihelion | the closest approach of a satellite to its primary; where it moves fastest in its orbit |
| gravity | attractive force between objects dependent on the masses of the objects and the distance between them |
| potential energy | energy of position |
| kinetic energy | energy of motion |
| astronomical unit (AU) | the average distance from the Earth to the Sun |
| meteors | small solid particles from space which burnup in the Earth's atmosphere |
| meteorite | meteors which reach the Earth |
| orbit | the path of a satellite around its primary |
| Copernicus | proposed heliocentric model of the solar system |
| Kepler | formulated three laws governing the motion of planets |
| heliocentric | sun-centered model of the solar system |
| ellipse | a closed curve around two fixed points known as the foci; all orbits are ellipses |
| focus (of an ellipse) | one of two fixed points that determine the shape of an ellipse |
| eccentricity | the degree of elongation of an ellipse; e=d/L |
