| A | B |
|---|
| escape velocity | the initial speed an object must have in order to free itself from the gravitational pull of a planet or other celestial body |
| force | a push or a pull exerted on an object |
| mass | a measure of the amount of matter an object contains; not dependent on the force of gravity |
| terminator | the line dividing the lighted portion of a nonluminous celestial body from the dark portion |
| ray | any of the bright streaks on the moon’s surface radiating from some of the moon’s craters |
| mare | a dark, flat lowland region on the moon’s surface |
| rill | long, narrow, snaking valleys especially evident on the moon’s surface |
| perigee | the point nearest the earth in the orbit of the moon or of an earth-orbiting object |
| apogee | the point in the orbit of the moon or man-made satellite where it is farthest from Earth |
| new moon | that phase of the moon when it is positioned in its orbit between the sun and the earth and is not visible because of the sun’s glare |
| waxing crescent | the increasing phase of the moon, during the week following a new moon, when its illuminated portion appears as a thin, gradually thickening crescent |
| cusp (lunar) | a pointed end of a crescent moon |
| earthshine | sunlight reflected from the dark side of the moon that was originally reflected from the earth |
| first quarter | the phase of the moon when the western half of the moon is lighted and the eastern half is dark |
| waxing gibbous | the increasing phase of the moon, the second week following the first quarter, when the illuminated portion is gradually enlarging toward a full moon |
| full moon | the lunar phase in which the moon’s entire near side is lighted |
| harvest moon | the full moon nearest the autumnal equinox (about September 22 in the Northern Hemisphere) |
| hunter’s moon | the next full moon after the harvest moon |
| waning gibbous | the decreasing phase of the moon, following a full moon, when the illuminated disk gradually diminishes to the third quarter |
| third quarter | same as last quarter; it is that phase of the moon in which the eastern half is lighted and the western half is dark |
| last quarter | same as third quarter; it is that phase of the moon in which the eastern half is lighted and the western half is dark |
| waning crescent | the decreasing phase of the moon during the last week of the lunar cycle |
| solar eclipse | an eclipse that occurs when the moon passes between the earth and the sun, blocking some or all of the sun’s light to the earth at a given location |
| total eclipse | an event where the direct (solar) or reflected (lunar) light from the sun as viewed by an observer at a given location is completely cut off by another astronomical body |
| partial eclipse | an eclipse in which the face of the sun is never fully covered, as observed at a particular location |
| annular eclipse | an eclipse of the sun in which a narrow ring (annulus) of the sun’s disk shows around the moon because the moon is too far from the earth to cover the sun’s disk completely |
| Bailey’s beads | bright pinpoints of light that appear briefly during a total solar eclipse as sunlight shines through valleys around the moon’s edge |
| diamond ring effect | during a total eclipse, the effect produced by a single Baily’s bead flash occurring along the thin remaining solar crescent at the limb of the moon; it occurs immediately before or after totality in a solar eclipse |
| lunar eclipse | the darkening of the full moon when it passes into the earth’s shadow |
| capture theory | the naturalistic theory that the moon was once a planet in its own orbit around the sun but that the earth somehow deflected it into an orbit around the earth |
| fission theory | a naturalistic theory of the origin of the moon suggesting that somehow the moon split away from the earth when the earth was still molten |
| accretion theory | the uniformitarian theory that all celestial objects, but especially those of the solar system, were formed by the gravitational accumulation of space dust |
| impact theory | a naturalistic theory for the origin of the moon that suggests the matter in the moon was once a part of earth, but a collision with another celestial object ejected the matter from the earth, and some of the matter coalesced and went into orbit around the earth as the moon |
