GLOSSARY

asteroids (AS tuh roids) — small bodies that orbit the Sun like planets

astronaut (AS troh nawt) — someone who travels in space

atmosphere (AT mohs feer) — the layer of gases surrounding the Earth

axis (AK sis) — an imaginary line on which a planet or other space object is said to rotate

comet (KAH meht) — a ball of ice and dust particles that orbits the Sun

dwarf planet (DWORF PLAN it) — a planet smaller than Mercury. Currently known dwarf planets include Pluto, Ceres, and Eris

eclipse (e KLIPS) — the blocking of the light of one celestial body, such as the Sun or the Moon, by another celestial body

galaxy (GAL ax see) — a system of stars held together by gravity

gas giant (GAS JIE uhnt) — one of the four large planets made mostly of gas. They include Jupiter, Saturn, Uranus, and Neptune

gravitational pull (grav ih TAY shun ul PUL) — the attraction one object exerts on another due to its mass

Kuiper Belt (KI per BELT) — a band of small bodies orbiting the Sun beyond Neptune

light year (LITE YEER) — the distance that light can travel in one year, or about 5.88 trillion miles (9.46 trillion km)

lunar phases (LOON er FAYZ es) — how the Moon appears to people on Earth

meteoroid (MEE tee uh roid) — a small piece of debris in the solar system

moon (MOON) — a natural satellite that orbits a planet

outer space (OW tur SPAYS) — the area of space beyond the Earth's atmosphere

planet (PLAN it) — any of the large bodies that orbit the Sun, including Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune in order of closeness to the Sun

solar system (SOH lur SIS tuhm) — the Sun and all the planets and objects that revolve around it

star (STAHR) — a large, luminous ball of gas

terrestrial planet (tuh REST ree uhl PLAN it) — a planet made mostly of rock. These include Mercury, Venus, Earth, and Mars

tide (TIDE) — the rising and falling of the oceans and other bodies of water

universe (YOO nih vers) — all the matter and energy in space