Sunrise

Sunrise or sun up is the instant at which the upper edge of the Sun appears over the horizon in the morning. The term can also refer to the entire process of the Sun crossing the horizon and its accompanying atmospheric effects.

Although the Sun appears to "rise" from the horizon, it is actually the Earth's motion that causes the Sun to appear. The illusion of a moving Sun results from Earth observers being in a rotating reference frame; this apparent motion is so convincing that most cultures had mythologies and religions built around the geocentric model, which prevailed until astronomer Nicklaus Copernicus first formulated the heliocentric model in the 16th century.

Architect Buck minster Fuller proposed the terms "sunsight" and "sunclipse" to better represent the heliocentric model, though the terms have not entered into common language.

Sunrise actually occurs before the Sun truly reaches the horizon because Earth's atmosphere refracts the Sun's image. At the horizon, the average amount of refraction is 34 arc minutes, though this amount varies based on atmospheric conditions.

Also, unlike most other solar measurements, sunrise occurs when the Sun's upper limb, rather than its center, appears to cross the horizon. The apparent radius of the Sun at the horizon is 16 arc minutes.

These two angles combine to define sunrise to occur when the Sun's center is 50 arc minutes below the horizon, or 90.83° from the zenith.