| A | B |
|---|
| geocentric | a description of a model of the solar system in which all of the planets revolve around Earth |
| heliocentric | a description of a model of the solar system in which Earth and other planets revolve around the sun |
| ecliptic plane | the plane in space containing Earth's orbit |
| moon | a relatively small natural body in space that revolves around a planet |
| astronomical unit | a unit of distance that equals average distance from Earth to the sun; 149,598,000 kilometers |
| space probe | an unpiloted vehicle that carries scientific instruments into space and transmits information back to earth |
| maria | teh low, flat plains of basalt on the moon formed by ancient lunar lava flow |
| crater | a round depression in the surface of a planet, moon, asteroid, or comet, caused by the impact of a meteroid |
| meteoroids | chunks of rock that move through the solar system |
| phases | the different shapes of the illuminated side of the moon as seen from Earth |
| eclipse | event that occurs when the shadow of one body in space, such as a planet or moon, falls on another |
| umbra | the darkest part of a shadow |
| penumbra | a region of the moon's shadow that surrounds the umbra and is less dark than the umbra |
| tides | the regular rise and fall of water in the oceans |
| spring tide | a tide at the new or full moon when the change between daily high and low tides is the greatest |
| neap tide | a tide at the first or last quarter moon, when there is the least change between daily high and low tides |
| terrestial planets | the four planets closest to the sun (Mercury, Venus, Earth, and Mars), whicha re similar ins tructure to Earth |
| asteroids | small, rocky solar-system bodies most of which are found orbiting the sun in a region between Mars and Jupiter |
| asteroid belt | region between the orbits of Mars and Jupiter where most asteroids in the solar system are found |
| gas giants | a large, massive planet ( such as Jupiter, Saturn, Uranus, or Neptune) that is composed mainly of hydrogen and helium |
| ring | a disk made of many small particles of rock and ice in orbit around a planet |
| Kuiper belt | a wide belt beyond Pluto's orbit where objects mostly made of ice and rock orbit the sun |
| Oort cloud | thought to be a very sparse sphere of comets encircling the solar system out to a distance of about 50,000 AU |
| solar nebula | a cloud of dust and gas that can eventually collapse to form one or more stars with a planetary system |
| protoplanetary disk | a large disk-shaped cloud of dust and gas formed as the solar nebula rotated faster and flattened out |
| planetesimals | asteroid-sized bodies in the protoplanetary disk from which the planets formed by accretion |
| accretion | the process of adding mass in the information of the planets by collisions between planetesimals |
