Sunrise - WikiHQ

thumb|Sunrise seen over the [[Atlantic Ocean through cirrus clouds on the Jersey Shore at Spring Lake, New Jersey, U.S.]]

Sunrise (or sunup) is the moment when the upper rim of the Sun appears on the horizon in the morning, at the start of the Sun path. The term can also refer to the entire process of the solar disk crossing the horizon.

Terminology

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 caused many cultures to have mythologies and religions built around the geocentric model, which prevailed until astronomer Nicolaus Copernicus formulated his heliocentric model in the 16th century.

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

Astronomically, sunrise occurs for only an instant, namely the moment at which the upper limb of the Sun appears tangent to the horizon. Civil twilight being the brightest, while astronomical twilight being the darkest.

Measurement

Angle with respect to horizon

thumb|This diagram of the Sun at [[false sunrise|sunrise (or sunset) shows the effects of atmospheric refraction.]]The stage of sunrise known as false 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 arcminutes, though this amount varies based on atmospheric conditions. Sunrises occur approximately due east on the March and September equinoxes for all viewers on Earth. Exact calculations of the azimuths of sunrise on other dates are complex, but they can be estimated with reasonable accuracy by using the analemma.

The figure on the right is calculated using the solar geometry routine in Ref. as follows:

For a given latitude and a given date, calculate the declination of the Sun using <math>0^{\circ}</math> longitude and solar noon time as inputs to the routine;
Calculate the sunrise hour angle using the sunrise equation;
Calculate the sunrise time, which is the solar noon time minus the sunrise hour angle in degree divided by 15;
Use the sunrise time as input to the solar geometry routine to get the solar azimuth angle at sunrise.

Hemispheric symmetry

An interesting feature in the figure on the right is apparent hemispheric symmetry in regions where daily sunrise and sunset actually occur.

This symmetry becomes clear if the hemispheric relation in to the sunrise equation is applied to the x- and y-components of the solar vector presented in Ref.

As a ray of white sunlight travels through the atmosphere to an observer, some of the colors are scattered out of the beam by air molecules and airborne particles, changing the final color of the beam the viewer sees. Because the shorter wavelength components, such as blue and green, scatter more strongly, these colors are preferentially removed from the beam. 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 other particles with diameters comparable to or larger than the sunlight's wavelengths (more than 600 nm) is due to Mie scattering and is not strongly wavelength-dependent. 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).

Sunset colors are typically more brilliant than sunrise colors, because the evening air contains more particles than morning air.

References

External links

Full physical explanation of sky color, in simple terms
An Excel workbook with VBA functions for sunrise, sunset, solar noon, twilight (dawn and dusk), and solar position (azimuth and elevation)
Sun data for various cities
Sunrise and sunset times in all popular cities