Sunrise or sun up is the instant at which the upper edge of the Sun appears on the horizon in the morning.1 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 Nicolaus Copernicus first formulated the heliocentric model in the 16th century.3
Architect Buckminster Fuller proposed the terms “sunsight” and “sunclipse” to better represent the heliocentric model, though the terms have not entered into common language.
Beginning and end
Astronomically, sunrise occurs for only an instant: the moment at which the upper limb of the Sun appears tangent to the horizon.1 However, the term sunrise commonly refers to periods of time both before and after this point:Twilight, the period during which the sky is light but the Sun is not yet visible (morning), or has just passed out of visibility (evening). The beginning of morning twilight is called dawn.The period after the Sun rises during which striking colors and atmospheric effects are still seen.
Sunrise occurs before the Sun actually reaches the horizon because the Sun’s image is refracted by the Earth’s atmosphere. The average amount of refraction is 34 arcminutes, though this amount varies based on atmospheric conditions.1
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 arcminutes.1
These two angles combine to define sunrise to occur when the Sun’s center is 50 arcminutes below the horizon, or 90.83° from the zenith.Read more