The Solar System

Front

Gravity Noun

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[grav-i-tee]

Back

Gravity

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A fundamental force of attraction that exists between any two objects with mass, pulling them toward each other.

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Example: This diagram shows gravity as a force, represented by arrows, that pulls everything towards the center of a massive object like the planet Earth.

Front

Inertia Noun

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[in-ur-shuh]

Back

Inertia

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The property of matter that causes an object to resist any change in its state of rest or motion.

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Example: The coin's inertia, its tendency to stay at rest, causes it to fall into the glass when the card is quickly removed.

Front

Geocentric Model Noun

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[jee-oh-sen-trik mod-l]

Back

Geocentric Model

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An astronomical model placing a stationary Earth at the center of the universe, with celestial bodies orbiting it.

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Example: This diagram shows the geocentric model, an early theory where the Earth is at the center and the Sun, Moon, and planets orbit it.

Front

Heliocentric Model Noun

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[hee-lee-oh-sen-trik mod-l]

Back

Heliocentric Model

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The astronomical model placing the Sun at the center of the solar system with the planets revolving around it.

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Example: This diagram shows the heliocentric model, where the Earth travels in an orbit around the Sun, which is at the center.

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 measurement equal to the average distance from the center of Earth to the center of the Sun.

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Example: This diagram shows that one Astronomical Unit (AU) is the distance from the Sun to Earth, and uses this unit to measure the distances to other planets.

Front

Space Probe Noun

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[speys prohb]

Back

Space Probe

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An unpiloted, instrument-laden vehicle sent into space to explore celestial bodies and transmit data back to Earth.

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Example: A space probe is an uncrewed spacecraft with key parts like a power source, science instruments, and a communication device to explore space.

Front

Maria Noun

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[mar-ee-uh]

Back

Maria

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The large, dark, flat plains on the Moon's surface that were formed by ancient, solidified lava flows.

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Example: This image identifies several maria, which are the large, dark, flat plains of cooled lava on the Moon's surface, historically mistaken for seas.

Front

Crater Noun

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[kray-ter]

Back

Crater

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A large, bowl-shaped depression on the surface of a planet or moon, caused by a high-speed meteoroid impact.

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Example: This diagram shows how a crater is formed when a projectile from space hits a surface, creating shockwaves and throwing out material called ejecta.

Front

Meteoroids Noun

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[mee-tee-uh-roidz]

Back

Meteoroids

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Chunks of rock or debris, smaller than asteroids, that travel through the solar system's interplanetary space.

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Example: This diagram shows the Asteroid Belt, a region of space between the orbits of Mars and Jupiter where most of the solar system's asteroids are found.

Front

Phases (of the moon) Noun

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[fay-zez]

Back

Phases (of the moon)

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The different shapes of the Moon's sunlit portion as seen from Earth, caused by its changing orbital position.

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Example: This diagram shows how the Moon's orbit around the Earth causes the sunlit portion we see to change, creating the different lunar phases.