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The moment at which a celestial body disappears below an observer's horizon. For bodies with an observable disk, particularly the Sun and Moon, setting is taken as the moment when their upper limb lies exactly on the observer's horizon. Refraction in the Earth's atmosphere means that an object appears to set later than it actually does, and this effect must be taken into account when calculating observed rising and setting times.

Subjects: Astronomy and Astrophysics.

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