| Term | Definition |
|---|
| astronomical unit | unit for measuring interplanetary distance equal to the average distance from the Earth t the Sun (150,000,000 km) |
| light-year | unit used to measure interstellar distances equal to the distance light travels in one year (9.5 trillion km) |
| ellipse | a "flattened" circle; the shape of planetary orbits |
| geocentric | the concept that the Earth is the center of the Solar System |
| heliocentric | the concept that the Sun is the center of the Solar System |
| terrestrial planet | small, rocky "Earth-like" planet |
| Jovian planet | large gaseous "Jupiter-like" planet |
| retrograde motion | apparent reversal of the motion of some planets as viewed from Earth; apparent backward motion |
| astronomy | study of the universe beyond the Earth |
| asteroid | a large fragment of rock the orbits the Sun |
| comet | an object of rock and ice that orbits the Sun |
| meteoroid | a small piece of dust, rock or metal left from a comet or asteroid collision |
| meteor | a bright streak of light that occurs when a meteoroid burns up in the Earth's atmosphere |
| meteorite | meteoroid or piece of a meteoroid left after it hits the Earth's surface |
| dwarf planet | small planetoids that orbit the Sun together beyond the orbit of Neptune |
| nebula | dark cloud of gas and dust in space |
| nebular theory | theory that the Sun, planets, and small members of the Solar System condensed and coalesced out of a cloud of dust and gas |
| galaxy | a large scale group of stars |
| Milky Way | the galaxy in which we live |
| star | a large ball of of gas that gives off radiant energy in the fofrm of heat and light |
| day | the time it takes for a planet to spin once on it axis |
| year | the time it takes for a planet to orbit the Sun once |
| satellite | a body that orbits another body of greater mass; a moon |
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