Horizon

The apparent boundary between Earth and sky, where celestial objects appear to rise or set. Its position depends on the observer’s location, elevation, and surroundings.

Geometric horizon: The boundary where a level plane through an observer’s eye meets the celestial sphere. It defines the ideal horizon from Earth’s curvature, before terrain or atmospheric effects intervene.
Altitude (astronomy): The angular height of a celestial object above or below an observer’s horizon. An object’s altitude reaches zero as it crosses the apparent horizon.
Celestial navigation: Navigation that determines position using measured angles between celestial bodies and the local horizon. Navigators use the horizon as the baseline for measuring a celestial body's altitude.
Event horizon: A boundary around a black hole beyond which events cannot send signals to a distant observer. Unlike the visible Earth–sky boundary, it is a causal limit rather than an optical appearance.
Dip of the horizon: The angle by which the visible sea horizon lies below an observer’s horizontal plane. Greater observer height makes the sea horizon appear lower.
Azimuth: The angle of a direction along the horizon, measured from a specified reference direction. Azimuth identifies where along the horizon an object rises or sets.
Sunrise: The daily event when the Sun’s upper edge appears above the horizon. Its observed time depends on the horizon’s elevation, obstruction, and atmospheric refraction.
Particle horizon: The farthest distance from which light could have reached an observer since the universe began. It is a cosmological limit on observable regions, not a local boundary in the sky.
Atmospheric refraction: The bending of light as it passes through layers of air with different densities. Refraction shifts the apparent horizon and lets the Sun remain visible near or just below it.
Earth's curvature: The gradual bending of Earth’s surface away from a local horizontal plane. Curvature causes the geometric horizon to appear at a finite distance.

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