| A | B |
|---|
| Accretion | The growth of planets from smaller objects by impact, one impact at a time. |
| Agglutinates | Common particle type in lunar sediment; it consists of small rock, mineral, and glass fragments bonded together with glass. |
| Asteroid | A small, mostly rocky body orbiting the Sun.These objects range in size from 1000 kilometers in diameter to tiny objects you could hold in your hand. Most of these things orbit the Sun between Mars and Jupiter, and are the source of most meteorites. |
| Carbon Star | A rare class of red giant stars unusually rich in carbon or carbon compounds. |
| Celestial Equator | An imaginary great circle in the sky drawn concentric to the Earth's equator. |
| Coma | A large cloud of dust and gas which escapes from the nucleus of an active comet. |
| Farside | The side of the moon that always faces away from Earth |
| Globular Clusters | Groups of thousands to millions of old stars held together in orbits about their own center of mass and arranged in almost symmetrical (spherical) clusters. |
| Heliocentric Distance | The distance from the sun |
| Interstellar Medium | The material between the stars, consisting of gas, dust and cosmic rays (high energy charged particles, moving at nearly the speed of light). |
| Long-period | In astronomy, a comet with an orbital period greater than 200 years. |
| Magnetosphere | The region around Earth dominated by Earth's magnetic field. In this region, the Earth's magnetic field lines guide the motions of charged particles (ions and electrons). |
| Maria (Mare) | Dark areas on the Moon covered by basalt lava flows. |
| Molecular Cloud | Large concentration of gas (roughly 75% hydrogen and 21-24% helium with trace amounts of other molecules), dust, and mineral grains. Our Solar System formed out of one approximately 4.55 billion years ago. |
| Moonquake | Sudden motion or trembling of the Moon caused by the abrupt release of slowly accumulated elastic energy in rocks. |
| Nautical Twilight | The time when the Sun is 6-12 degrees below the horizon. Sky begins to get noticeably bright. |
| Nearside | The side of the moon that always faces the Earth |
| Protostar | A star in the process of formation which has not yet become hot enough in the core to initiate the process of nuclear fusion (107 K) to halt its gravitational collapse. |
| Rille | Long channel on the Moon crossing the surface of maria; probably formed either as an open channel in a lava flow, or as an under-ground tube carrying hot lava which collapsed as the lava flowed out. |
| Short-period | In astronomy, a comet with an orbital period less than 200 years. |
| Solar Flare | A region of exceptional brightness in an atmospheric layer of the Sun, often associated with sunspots of complex magnetic fields. |
| Solar Nebula | The immense cloud of gas and particles of floating cosmic dust from which our Solar System (Sun and planets) condensed 4.6 billion years ago. |
| Solar Wind | The stream of charged particles (mainly ionized hydrogen) moving outward from the Sun with velocities in the range 300-500 kilometers per second. |
| Tidal Stresses | Force per unit area acting on a planetary body resulting in periodic bulging (of the crust and, in the case on Earth, oceans) caused by the gravitational attraction of another object such as the Sun, a moon, or a planet. The alternate growth and decay of a tide in the crust of a planetary body can lead to frictional heating. |
| Troctolite | Igneous rock found in the lunar highlands composed of plagioclase and olivine. |
