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horizon
- Astronomical horizon (or sensible horizon): the great circle formed by the intersection of the celestial sphere with a plane perpendicular to the line from an observer to the zenith; in other words, the great circle whose poles are the nadir and zenith.
- Cosmological horizon: the maximum distance from us that light has traveled since the beginning of the universe; objects farther away are invisible to us because there has not been enough time for light to travel from them to Earth.
Related categories
• CELESTIAL MECHANICS
• COSMOLOGY
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