| A | B |
|---|
| asteroid | fragment of rock that orbits the sun |
| asteriod belt | the region between the orbits of Mars and Jupiter in which most asteroids are found |
| astronomical unit | average distance between the earth and the sun |
| coma | spherical cloud of gas and dust that surrounds the nucleus of a comet |
| comet | body of rock, dust, methane, ammonia, and ice that revolves around the sun in a long, elliptical, orbit |
| focus | one of two points within an ellipse that determines the shape of the figure |
| geocentric | earth-centered |
| heliocentric | sun-centered |
| Jovian planet | any one of the first four oute planets- Jupiter, Saturn, Uranus, or Neptune- with properties similar to those of Jupiter |
| meteor | meteoroid that enters the earth's atmosphere |
| meteor shower | phenomenon caused by the burning up of large numbers of meteors as they enter the earth's atmosphere |
| meteorite | meteor or part of a meteor left after it hits the earth's surface |
| meteoroid | small bit of rock or metal moving through the solar system left by a comet or produced by a collision between asteroids |
| orbit period | time required for a planet to make one revolution around the sun |
| terrestrial planets | any one of the four planets closest to the sun- Mercury, Venus, Earth, and Mars- with properties similar to those of the earth |
| apogee | point in the orbit of a satellite at which the satellite is the farthest from the earth |
| crater | bowl-shaped depression on the surface of a planetary body |
| eclipse | passing of one planetary body through the shadow of another |
| full moon | phase of the moon during which the entire half of the moon facing the earth is visible |
| Galilean moon | any one of the four largest satellites of Jupiterm wgug were first seen by Galileo |
| lunar eclipse | passing of the earth between the moon and the sun during which the earth's shoadow crosses the lighted half of the moon |
| maria | dark areas of smooth, dry, solidified lava on the moon that reflect little light |
| new moon | phase of the moon during which the side of the moon facing the earth is unlighted |
| penumbra | outer part of the shadow cast by the earth or the moon in which sunlight is only partially blocked |
| perigee | point in the orbit of a satellite at which it is closest to the earth |
| phase | varying shape of the visible portion of the moon |
| rille | long, deep channel that runs through the maria on the moon |
| solar eclipse | passing of the moon between the earth and the sun during which the shadow of the moon falls on the earth |
| solar year | time required for the earth to make one orbit around the sun, about 365.24 days |
| umbra | inner, cone-shapped part of the shadow cast by the earth or the moon in which sunlight is completely blocked |
| waning | describing the phase of the moon during which the size of its visible porion is decreasing |
| waxing | describing the phase of the moon during which the size of its visible portion is increasing |
