| A | B |
|---|
| astronomical unit | The average distance between the Sun and Earth, which is 1.496 x 10^8 |
| eccentricity | Ratio of the distance between the foci to the length of the major axis; defines the shape of a planet’s elliptical orbit |
| gas giant | Large, gaseous planets are very cold at their surfaces, have ring systems, many moons, and lack solid surfaces |
| terrestrial giant | Rocky-Surfaced, relatively small, dense inner planets closest to the Sun-Mercury, Venus, Earth, and Mars |
| asteroid | Metallic or silica-rich object, 1 km to 950 km in diameter that bombarded early Earth generating heat energy; rocky remnant of the early solar system found mostly between the orbits of Mars and Jupiter in the asteroid belt |
| comet | Small, eccentrically orbiting body made of rock and ice that consists of a nucleus, a coma, and one or more tails that point away from the Sun |
| meteor | Streak of light produced when a meteoroid falls toward Earth and burns up in the Earth’s atmosphere |
| meteorite | Small asteroid or asteroid fragment that fell on Earth, generating heat; meteoroid that does not completely burn up in the Earth’s atmosphere and strikes Earth’s surface, sometimes causing an impact crater |
| meteoroid | Piece of interplanetary material that falls toward Earth and enters its atmosphere |
| planetesimal | Space object built of solid particles that can form through collisions and mergers |
| retrograde motion | When an object moves or appears to move backwards |
| geocentric | Earth centered |
| heliocentric | Sun centered |
