Unit 21

Highlands- Light-colored, mountainous, heavily cratered area of the Moon composed mostly of lunar breccias.

Maria- Dark-colored, smooth planes on the Moon’s surface.

Rilles- Valleylike structure that meanders across some regions of the Moon’s maria.

Impact craters- Crater formed when space material crashes into the surface of a celestial body.

Ejecta- Material that falls back to the lunar surface after being blasted out by the impact of a space object.

Rays- Long trail of ejecta that radiates outward from an impact crater.

Regolith- Layer of loose, ground-up rock on the lunar surface.

Ecliptic Plane- Plane of Earth’s orbit around the Sun.

Solstice- Period when the Sun is overhead at its farthest distance either north or south of the equator.

Equinox- Time of year during which Earth’s axis at a 90-degree angle to the Sun; both hemispheres receive exactly 12 hours of sunlight, and the Sun is directly overhead at the equator.

Synchronous Rotation- The state at which an orbiting body’s orbital and rotational periods are equal.

Solar Eclipse- When moon passes between Earth and the Sun and the Moon casts a shadow on Earth, blocking Earth’s view of the Sun; can be partial or total.

Perigee- Closest point in the Moon’s elliptical orbit to Earth.

Apogee- Farthest point in an object’s orbit to Earth.

Lunar Eclipse- When Earth passes between the Sun and the Moon, and Earth’s shadow falls on the Moon; occurs only during a full moon.