Constellations

Front

Constellation Noun

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[kon-stuh-ley-shuhn]

Back

Constellation

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A group of stars forming a recognizable pattern that has been named, occupying a region with well-defined boundaries.

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Example: This image shows the constellation Leo by connecting its stars to form a pattern and overlaying the mythological figure of a lion it represents.

Front

Celestial Sphere Noun

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[suh-les-chul sfeer]

Back

Celestial Sphere

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An imaginary sphere of infinite radius with the Earth at its center, on which all celestial objects are considered to lie.

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Example: This diagram shows the celestial sphere, an imaginary globe surrounding Earth, used to map stars and constellations. It labels key reference points like the celestial equator and poles.

Front

Celestial Equator Noun

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[suh-les-chul ih-kwey-ter]

Back

Celestial Equator

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An imaginary great circle on the celestial sphere, lying in the same plane as the Earth's equator.

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Example: This diagram shows how the Celestial Equator is an imaginary line in space, created by projecting Earth's own equator onto the celestial sphere.

Front

Celestial Pole Noun

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[suh-les-chul pohl]

Back

Celestial Pole

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The point on the celestial sphere directly above either of the Earth's geographic poles, around which stars appear to rotate.

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Example: This diagram shows that the celestial poles are the points in space directly above Earth's North and South poles, along its extended axis of rotation.

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.

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Example: This diagram shows the zenith as the point on the celestial sphere directly above an observer, with the horizon as the line where the sky meets the ground.

Front

Nadir Noun

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[ney-der]

Back

Nadir

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The point on the celestial sphere directly below an observer, opposite the zenith.

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Example: The image shows that for an observer on a celestial body, the nadir is the direction pointing straight down, directly away from the sky.

Front

Astronomical Horizon Noun

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[as-truh-nom-i-kuhl huh-rahy-zuhn]

Back

Astronomical Horizon

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The boundary where the sky appears to meet the ground from the perspective of an observer.

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Example: This diagram shows that the astronomical horizon is an imaginary flat plane extending from an observer's eyes, different from the true horizon which is curved by the Earth.

Front

Rotation Noun

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[roh-tey-shuhn]

Back

Rotation

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The turning motion of a celestial body on its axis, causing the apparent daily movement of stars across the sky.

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Example: The Earth spins on its tilted axis, which is called rotation. This movement causes the daily cycle of day (facing the Sun) and night (in shadow).

Front

Revolution Noun

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[rev-uh-loo-shuhn]

Back

Revolution

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The orbital motion of a celestial body around another, which causes seasonal changes in visible constellations.

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Example: This diagram shows the Earth's revolution, its year-long orbit around the Sun, which causes the different seasons due to the planet's constant axial tilt.

Front

Orbit Noun

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[awr-bit]

Back

Orbit

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The curved, typically elliptical, path of a celestial object or spacecraft around a star, planet, or moon.

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Example: This diagram shows a smaller celestial body, like a planet, following a curved path called an orbit around a larger central body, like a star.