| Term | Definition |
| asteroid | small, rocky space object found in the asteroid belt between the orbits of Jupiter and Mars |
| astronomical unit | unit of measure used to determine distances between objects in the Solar System; 1 AU equals 150,000,000 km. |
| axis | imaginary line around which earth spins, causing day and night, and that is drawn from the north geographic pole through earth to the south geographic pole |
| comet | space object made of rocky particles and water ices; forms a tails when orbiting near the sun and is found mostly in the Kuiper Belt and the Oort Cloud |
| crater | depression formed by a large meteorite; the more craters in a region, the older the surface |
| equinox | twice-yearly time when the sun is directly above the earth's equator and the length of the day equals the length of the night worldwide |
| lunar eclipse | occurs during a full moon, the sun, the moon, and earth line up in a specific way and the moon moves into earth's shadow |
| moon phase | changing views of the moon as seen from earth, which are caused by the moon's revolutiion around earth |
| nebula | cloud of rotating gases and dust particles from which the solar system ma have formed about 5 billion years ago |
| orbit | curved path followed by earth around the sun |
| revolution | the motion of earth around the sun, which takes about 365 1/4 days, or one year to complete |
| rotation | spinning of earth on its axis, which causes day and night; it takes 24 hours for earth to complete one rotation |
| solar eclipse | occurs during a new moon, when the sun, the moon, and earth are lined up in a specific way and earth moves into the moon's shadow |
| solar system | extremely large system that includes th esun, planets, comets, meteoroids, and other objects, that orbit the sun |
| solstice | time when sun reaches its greatest distance north or south of the equator, which occurs June 21 or 22 for the northern hemispere(Longest day of the year) and December 21 or 22 for the southern hemisphere (shortest day of the year) |