Sunrise or sun up is the instant at which the upper edge of the Sun appears over the eastern 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 Nicolaus Copernicus first formulated the heliocentric model in the 16th century.
Sunrise occurs before the Sun actually reaches the horizon because the Sun’s image is refracted by the Earth’s atmosphere. At the horizon, the average amount of refraction is 34 arcminutes, though this amount varies based on atmospheric conditions.
“What sunshine is to flowers, smiles are to humanity. These are but trifles, to be sure; but scattered along life’s pathway, the good they do is inconceivable.”
― Joseph Addison
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.
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.The timing of sunrise varies throughout the year and is also affected by the viewer’s longitude and latitude, altitude, and time zone. These changes are driven by the axial tilt of Earth, daily rotation of the Earth, the planet’s movement in its annual elliptical orbit around the Sun, and the Earth and Moon’s paired revolutions around each other. The analemma can be used to make approximate predictions of the time of sunrise.Sunrise occurs before the Sun actually reaches the horizon because the Sun’s image is refracted by the Earth’s atmosphere.
At the horizon, the average amount of refraction is 34 arcminutes, though this amount varies based on atmospheric conditions. Although the Sun appears to «rise» from the horizon, it is actually the Earth’s motion that causes the Sun to appear.