The Universe

Front

Solar System Noun

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[soh-ler sis-tuhm]

Back

Solar System

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The system of the sun and the celestial objects, including planets and moons, that are held in its gravitational pull.

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Example: This diagram shows the Sun at the center with planets, including Earth, orbiting around it in paths called orbits, which together form the Solar System.

Front

Gravity Noun

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[grav-i-tee]

Back

Gravity

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The universal force of attraction between all objects with mass, which keeps planets and other celestial bodies in their orbits.

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Example: This diagram shows that gravity is a force that pulls objects toward the center of a massive body, like the planet Earth.

Front

Orbit Noun

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[or-bit]

Back

Orbit

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The curved path a celestial object takes as it revolves around another object due to gravitational attraction.

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Example: This diagram shows the Earth moving in a curved path, called an orbit, around the Sun.

Front

Astronomical Unit (AU) Noun

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[as-truh-nom-i-kuhl yoo-nit]

Back

Astronomical Unit (AU)

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A unit of length used for measuring astronomical distances, equal to the average distance from Earth to the Sun.

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Example: This diagram shows that an Astronomical Unit (AU) is the average distance between the Earth (a planet) and the Sun (a star).

Front

Planet Noun

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[plan-it]

Back

Planet

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A large, round celestial body that orbits a star and has cleared its orbital path of other significant debris.

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Example: This image shows several planets, which are large celestial bodies, moving in fixed paths called orbits around a central star, the Sun.

Front

Rotation Noun

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[roh-tey-shuhn]

Back

Rotation

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The action of a celestial body, such as a planet, turning or spinning on its axis, which causes day and night.

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Example: The Earth spins, or rotates, on an imaginary line called an axis. This rotation is what causes the cycle of day and night.

Front

Axis Noun

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[ak-sis]

Back

Axis

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An imaginary line passing through the center of a celestial body, such as a planet, around which it rotates.

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Example: This diagram shows Earth's axis, the imaginary line it rotates around, which is tilted at an angle of 23.45 degrees relative to its orbit.

Front

Dwarf Planet Noun

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[dworf plan-it]

Back

Dwarf Planet

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A celestial body that orbits the sun and is spherical, but has not cleared the area of its orbit of debris.

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Example: This image displays the five officially recognized dwarf planets, including Pluto, showing that they are round celestial bodies found in our solar system.

Front

Natural Satellite Noun

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[nach-er-uhl sat-l-ahyt]

Back

Natural Satellite

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A celestial body, commonly known as a moon, that is in a fixed orbit around a planet or smaller body.

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Example: The Moon is a natural satellite because it is a celestial body that orbits a larger body, the Earth, held in place by gravity.

Front

Asteroid Noun

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[as-tuh-roid]

Back

Asteroid

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A small, rocky body that orbits the sun, with many being found in the asteroid belt between Mars and Jupiter.

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Example: This image shows an asteroid, a rocky, irregularly shaped object that orbits the Sun. Its cratered surface reveals a history of collisions in space.