Earth, Moon, and Sun System

Front

Rotation Noun

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[roh-tey-shuhn]

Back

Rotation

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The spinning of a celestial body, such as a planet, on its own internal axis, causing day and night.

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Example: The Earth spins on its tilted axis, which is called rotation; this movement causes the cycle of day (illuminated side) and night (shadowed side).

Front

Revolution Noun

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[rev-uh-loo-shuhn]

Back

Revolution

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The motion of one celestial body as it travels in an orbit around another body or a point in space.

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Example: This diagram shows the Earth's revolution, its year-long elliptical orbit around the Sun, highlighting the points where it is closest (perihelion) and farthest (aphelion).

Front

Axis Noun

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[ak-sis]

Back

Axis

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An imaginary straight line passing through the center of a body, around which the body rotates or spins.

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Example: This diagram shows Earth tilted on its axis, an imaginary line that it spins around, which causes day and night.

Front

Orbit Noun

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[awr-bit]

Back

Orbit

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The curved, often elliptical, path that a celestial body or spacecraft takes as it revolves around another body.

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Example: This diagram shows the Moon's orbit, a curved path around the Earth, caused by the balance between the Moon's forward velocity and Earth's gravitational pull.

Front

Precession Noun

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[pri-sesh-uhn]

Back

Precession

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The slow, conical wobble of a rotating body's axis, like the 26,000-year cycle of Earth's rotational axis.

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Example: Due to Earth's slow wobble, called precession, its axis points to different 'North Stars' over a 26,000-year cycle, tracing a circle in the sky.

Front

Mean Solar Day Noun

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[meen soh-ler dey]

Back

Mean Solar Day

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The average time interval, approximately 24 hours, from one noon to the next, based on the sun's position.

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Example: A solar day (24 hours) is the time for Earth to rotate and face the Sun again. It's longer than a 360° rotation because Earth also orbits the Sun.

Front

Zenith Noun

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[zee-nith]

Back

Zenith

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The point on the celestial sphere that is directly above an observer on Earth; the highest point in the sky.

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Example: The diagram shows an observer on a horizon plane inside a celestial sphere, with the Zenith labeled as the point directly overhead.

Front

Sidereal Time Noun

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[sahy-deer-ee-uhl tahym]

Back

Sidereal Time

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A timekeeping system based on Earth's rotation relative to the fixed stars rather than the sun.

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Example: A sidereal day is Earth's true 360° rotation (23h 56m). A solar day is longer (24h) because Earth must rotate slightly more to face the Sun again.

Front

Plane of the Ecliptic Noun

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[pleyn uhv thuh ih-klip-tik]

Back

Plane of the Ecliptic

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The imaginary flat surface that contains Earth's orbital path as it revolves around the sun.

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Example: This diagram shows the Earth's orbital path, the Plane of the Ecliptic, is tilted compared to its rotational axis, which is projected as the celestial equator.

Front

Season Noun

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[see-zuhn]

Back

Season

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A division of the year marked by distinct weather patterns, resulting from Earth's axial tilt and its revolution.

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Example: The Earth's tilted axis causes seasons. As Earth orbits the Sun, different parts receive more direct sunlight, leading to summer, autumn, winter, and spring.