| A | B |
|---|
| aphelion | the place in the orbit of a planet where the planet is farthest from the sun |
| apogee | the point where the moon is farthest from Earth |
| astronomical unit (AU) | average distance from Earth to the sun; 1.5 × 108, or 150 million kilometers |
| astronomy | the scientific study of the universe; It includes the observation and interpretation of celestial bodies and phenomena. |
| crater | the depression at the summit of a volcano or that which is produced by a meteorite impact |
| ellipse | an oval |
| geocentric | describes the concept of an Earth-centered universe |
| heliocentric | describes the view that the sun is at the center of the solar system |
| lunar eclipse | an eclipse of the moon; occurs when the moon passes through Earth’s shadow. |
| lunar regolith | a thin, gray layer on the surface of the moon, consisting of loosely compacted, fragmented material believed to have been formed by repeated impacts of meteorites |
| mare | the Latin name for the smooth areas of the moon formerly thought to be seas |
| perigee | the point at which the moon is closest to Earth |
| perihelion | the point in the orbit of a planet where it is closest to the sun |
| phases of the moon | the progression of changes in the moon’s appearance during the month |
| precession | a slow motion of Earth’s axis that traces out a cone over a period of 26,000 years |
| ray | any of a system of bright elongated streaks, sometimes associated with a crater on the moon |
| retrograde motion | the apparent westward motion of the planets with respect to the stars |
| revolution | the motion of one body about another, as Earth about the sun |
| rille | long channel associated with lunar maria; A rille looks similar to a valley or a trench. |
| rotation | the spinning of a body, such as Earth, about its axis |
| solar eclipse | an eclipse of the sun;occurs when the moon moves in a line directly between Earth and the sun, casting a shadow on Earth. |
