AST-006beginner

Astronomical Units, Light-Years & Parsecs: Foundations

Astronomers use different distance units because the universe spans scales too large for convenient everyday units. Astronomical units are natural for Solar-System orbits, light-years are intuitive for light-travel time, and parsecs are convenient for stellar and galactic astronomy because they connect directly to parallax and common astronomical distance scales.

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Last reviewed9/21/2026

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Astronomical Units, Light-Years & Parsecs: Foundations

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AST-005Scale of the Universe: Advanced Connections→

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AST-007Astronomical Units, Light-Years & Parsecs: Structures & Vocabulary→

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Astronomers use different distance units because the universe spans scales too large for convenient everyday units.

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A parsec comes from parallax geometry: as Earth moves from one side of its orbit to the other, a nearby star appears to shift against distant background stars.

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