Sunrise

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Sunrise or sun up is the instant at which the upper edge of the Sun appears on 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. http://www.merriam-webster.com/dictionary/sunrise

Terminology

"Rise"

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 for over 1500 years until astronomer Nicolaus Copernicus first formulated the heliocentric model in the 16th century. http://science.discovery.com/top-ten/2009/science-mistakes/science-mistakes-02.html

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. 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. The beginning of twilight is called dawn .

The period after sunrise during which striking colors and atmospheric effects are still seen.

Measurement

Angle

Sunrise occurs before the Sun actually reaches the horizon because the Sun's image is atmospheric refraction|refracted by the Earth's atmosphere. The average amount of refraction is 34 arcminute s, 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 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 .

Time of day

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 ellipse|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 the late winter and spring, sunrise as seen from temperate latitudes occurs earlier each day, reaching its earliest time near the summer solstice ; the exact date varies by latitude. After this point, the sunrise time 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 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 apparent position. Near the poles, the time-of-day variation is exaggerated, since the Sun crosses the horizon at a very shallow angle and thus rises more slowly.

Accounting for atmospheric refraction and measuring from the leading edge slightly increases the average duration of daylight|day relative to night . The sunrise equation , however, which is used to derive the time of sunrise and sunset, uses the Sun's physical center for calculation, neglecting atmospheric refraction and the non-zero angle subtended by the solar disc.

Location on the horizon

Due to Earth's axial tilt, 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.

Appearance

Colors

Air molecules and Aerosol|airborne particles scatter white sunlight as it passes through the Earth's atmosphere. This is done by a combination of Rayleigh scattering and Mie scattering .

Rayleigh scattering by smaller particles

Pure sunlight is white in color, containing a spectrum of colors from violet to red. When sunlight interacts with atmospheric particles much smaller than the wavelength of visible light , a phenomenon known as Rayleigh scattering occurs. In this process, light is scattered in various directions, with shorter wavelengths (violet, blue, and green) being scattered more strongly than longer ones (orange and red).Craig Bohren (ed.), Selected Papers on Scattering in the Atmosphere , SPIE Optical Engineering Press, Bellingham, WA, 1989

Because of this effect, the Sun generally appears yellow when observed on Earth, since some of the shorter wavelengths are scattered into the surrounding sky. This also makes the sky appear increasingly blue farther away from the Sun. During sunrise and sunset, the longer path through the atmosphere results in the removal of even more violet and blue light from the direct rays, leaving weak intensities of orange to red light in the sky near the Sun.

Mie scattering by larger particles

After Rayleigh scattering has removed the violets and blues from the direct rays, the remaining reddened sunlight can then be scattered by cloud droplets and other relatively large particles to light up the horizon red and orange. These larger particles, with sizes comparable to and longer than the wavelength of light, scatter light by mechanisms treated by the Mie theory .

Mie scattering does not depend heavily on wavelength, but it has the largest effect when an observer views the light directly (such as toward the Sun), rather than looking in other directions. Mie scattering is responsible for the light scattered by clouds, and also for the daytime halo of white light around the Sun (forward scattering of white light).

Without Mie scattering at sunset and sunrise, the sky along the horizon has only a dull-reddish appearance, while the rest of the sky remains mostly blue and sometimes green.

Ash from volcanic eruptions, trapped within the troposphere , tends to mute sunset and sunrise colors, whereas volcanic ejecta lofted into the stratosphere (as thin clouds of tiny sulfuric acid droplets) can yield beautiful post-sunset colors called afterglow s 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.

Sunrise vs. Sunset colors

Sunset colors are sometimes more brilliant than sunrise colors because evening air typically contains more large particles, such as clouds and smog, than morning air. These particles glow orange and red due to Mie scattering during sunsets and sunrises because they are illuminated with the longer wavelengths that remain after Rayleigh scattering.Selected Papers on Scattering in the Atmosphere, edited by Craig Bohren ~SPIE Optical Engineering Press, Bellingham, WA, 1989

If the concentration of large particles is too high (such as during heavy smog), the color intensity and contrast is diminished and the lighting becomes more homogenous. When very few particles are present, the reddish light is more concentrated around the Sun and is not spread across and away from the horizon.

Optical illusions and other phenomena

Atmospheric refraction causes the Sun to be seen while it is still below the horizon.

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 is 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| false sunrise occurs, demonstrating a very 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.

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