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 additionally refer to the entire process of the Sun crossing the horizon and its accompanying atmospheric effects. The Sun will rise exactly due east at the spring and fall equinoxes, which each occur only once a year.
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 obvious 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 sixteenth century.
Architect Buckminster Fuller proposed the terms "sunsight" and "sunclipse" to better represent the heliocentric model, though the terms haven't 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. Notwithstanding the term sunrise commonly refers to periods of time both before and after this point:
- Twilight, the period in the morning throughout which the sky is light but the Sun isn't yet visible. The beginning of morning twilight is called dawn.
- The period after the Sun rises throughout which striking colours 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. At the horizon, the average amount of refraction is 34 arcminutes, though this amount varies based on atmospheric conditions.
Also, unlike most additional solar measurements, sunrise occurs when the Sun's upper limb, rather than its center, appears to cross the horizon. The obvious radius of the Sun at the horizon is 16 arcminutes.
These two angles combine to define sunrise to occur when the Sun's centre is 50 arcminutes below the horizon, or 90.83° from the zenith.
Time of day
The timing of sunrise varies throughout the year and is additionally 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.
In late winter and spring, sunrise as seen from temperate latitudes occurs earlier each day, reaching its earliest time near the summer solstice; although the exact date varies by latitude. After this point, the time of sunrise gets later each day, reaching its latest sometime around the winter solstice. The offset between the dates of the solstice and the earliest or latest sunrise time is caused by the eccentricity of Earth's orbit and the tilt of its axis, and is described by the analemma, which can be used to predict the dates.
Variations in atmospheric refraction can alter the time of sunrise by changing its obvious position. Near the poles, the time-of-day variation is exaggerated, after the Sun crosses the horizon at a quite shallow angle and thus rises more slowly.
Accounting for atmospheric refraction and measuring from the leading edge slightly increases the average duration of day relative to night. The sunrise equation, however, which is used to derive the time of sunrise and sunset, uses the Sun's physical centre for calculation, neglecting atmospheric refraction and the non-zero angle subtended by the solar disc.
Location on the horizon
Neglecting the effects of refraction and the Sun's non-zero size, whenever and wherever sunrise occurs, it is always in the northeast quadrant from the March equinox to the September equinox and in the southeast quadrant from the September equinox to the March equinox. Sunrises occur due east on the March and September equinoxes for all viewers on Earth. Exact calculations of the azimuths of sunrise on additional dates are complex, but they can be estimated with reasonable accuracy by using the analemma.
As a ray of white sunlight travels through the atmosphere to an observer, a few of the colours are scattered out of the beam by air molecules and airborne particles, changing the final colour of the beam the viewer sees. Because the shorter wavelength components, such as blue and green, scatter more strongly, these colours are preferentially removed from the beam. At sunrise and sunset, when the path through the atmosphere is longer, the blue and green components are removed almost completely leaving the longer wavelength orange and red hues we see at those times. The remaining reddened sunlight can then be scattered by cloud droplets and additional relatively large particles to light up the horizon red and orange. The removal of the shorter wavelengths of light is due to Rayleigh scattering by air molecules and particles much smaller than the wavelength of visible light (less than 50 nm in diameter). The scattering by cloud droplets and additional particles with diameters comparable to or larger than the sunlight's wavelengths (> 600 nm) is due to Mie scattering and isn't strongly wavelength-dependent. Mie scattering is responsible for the light scattered by clouds, and additionally for the daytime halo of white light around the Sun (forward scattering of white light).
Sunset colours are typically more brilliant than sunrise colors, because the evening air contains more particles than morning air.
Ash from volcanic eruptions, trapped within the troposphere, tends to mute sunset and sunrise colors, while volcanic ejecta that's instead lofted into the stratosphere (as thin clouds of tiny sulfuric acid droplets), can yield beautiful post-sunset colours called afterglows and pre-sunrise glows. A number of eruptions, including those of Mount Pinatubo in 1991 and Krakatoa in 1883, have produced sufficiently high stratospheric sulfuric acid clouds to yield remarkable sunset afterglows (and pre-sunrise glows) around the world. The high altitude clouds serve to reflect strongly reddened sunlight still striking the stratosphere after sunset, down to the surface.
Optical illusions and additional phenomena
- Atmospheric refraction causes the Sun to be seen while it is still below the horizon.
- Light from the lower edge of the Sun's disc is refracted more than light from the upper edge. This reduces the obvious height of the Sun when it appears just above the horizon. The width isn't affected, so the Sun appears wider than it is high.
- The Sun appears larger at sunrise than it does while higher in the sky, in a manner similar to the moon illusion.
- The Sun appears to rise above the horizon and circle the Earth, but it is actually the Earth that's rotating, with the Sun remaining fixed. This effect results from the fact that an observer on Earth is in a rotating reference frame.
- Occasionally a false sunrise occurs, demonstrating a quite particular kind of Parhelion belonging to the optical phenomenon family of halos.
- Sometimes just before sunrise or after sunset a green flash can be seen. This is an optical phenomenon in which a green spot is visible above the sun, usually for no more than a second or two.