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.