The apparent westward revolution of Sun around the earth after rising out of the horizon is due to the Earth's eastward rotation, a counter-clockwise revolution when viewed from above the North Pole. This illusion is so convincing that most cultures had mythologies and religions built around the geocentric model. This same effect can be seen with near-polar satellites as well.
Sunrise and sunset are calculated from the leading and trailing edges of the Sun, and not the center; this slightly increases the duration of "day" relative to "night". The sunrise equation, however, is based on the center of the sun.
The timing of sunrise varies throughout the year and is also affected by the viewer's longitude and latitude, altitude and time zone. Small daily changes and noticeable semi-annual changes in the timing of sunrises 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. In the summertime, the days get longer and sunrises occur earlier every day until the day of the earliest sunrise, which occurs before the summer solstice. In the Northern Hemisphere, the earliest sunrise does not fall on the summer solstice around June 21, but occurs earlier in June. The precise date of the earliest sunrise depends on the viewer's latitude (connected with the slower Earth's movement around the aphelion around July 4). Similarly, the latest sunrise does not occur on the winter solstice, but rather about two weeks later, again depending on the viewer's latitude. In the Northern Hemisphere the latest sunrise occurs in early January (influence from the Earth's faster movement near the perihelion which occurs around January 3). Likewise, the same phenomena exist in the Southern Hemisphere except with the respective dates reversed, with the latest sunrises occurring some time after June 21 in winter and earliest sunrises occurring some time before December 21 in summer, again depending on one's southern latitude. For one or two weeks surrounding both solstices, both sunrise and sunset get slightly later or earlier each day. Even on the equator, sunrise and sunset shift several minutes back and forth through the year, along with solar noon. These effects are plotted by an analemma.
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.
Rayleigh scattering is the elastic scattering of electromagnetic radiation due to the polarizability of the electron cloud in molecules and particles much smaller than the wavelength of visible light. Rayleigh scattering intensity is fairly omnidirectional and has a strong reciprocal 4th-power wavelength dependency and, thus, the shorter wavelengths of violet and blue light are affected much more than the longer wavelengths of yellow to red light. During the day, this scattering results in the increasingly intense blue color of the sky away from the direct line of sight to the Sun, while during sunrise and sunset, the much longer path length through the atmosphere results in the complete removal of violet, blue and green light from the incident rays, leaving weak intensities of orange to red light. After Rayleigh scattering has removed the violets, blues, and greens, people's viewing of red and orange colors of sunsets and sunrises is then enhanced by the presence of particulate matter, dust, soot, water droplets (like clouds), or other aerosols in the atmosphere, (notably sulfuric acid droplets from volcanic eruptions). Particles much smaller than the wavelength of the incident light efficiently enhance the blue colors for off-axis short path lengths through air (resulting in blue skies, since Rayleigh scattering intensity increases as the sixth power of the particle diameter). Larger particles as aerosols, however, with sizes comparable to and longer than the wavelength of light, scatter by mechanisms treated, for spherical shapes, by the Mie theory. Mie scattering is largely wavelength insensitive. Its spacial distribution is highly preferential in the forward direction of the incident light being scattered, thus having its largest effect when an observer views the light in the direction of the rising or setting Sun, rather than looking in other directions. During the daytime, Mie Scattering generally causes a diffuse white halo around the Sun decreasing the perception of blue color in the direction toward the Sun and it causes daytime clouds to appear white due to white sunlight. At sunset and sunrise, Mie scattering off of particles and aerosols across the horizon, then transmits the red and orange wavelengths that remain after Rayleigh scattering has depleted the blue light. This explains why sunsets without soot, dust, or aerosols are dull and fairly faint red, while sunsets and sunrises are brilliantly intense when there are lots of soot, dust, or other aerosols in the air.
Sunset colors are typically more brilliant than sunrise colors, because the evening air contains generally more particles and aerosols and clouds than morning air. Cloud droplets are much larger than the wavelength of light; so they scatter all colors equally by Mie scattering, which makes them appear white when illuminated by white sunlight during the daytime. The clouds glow orange and red due to Mie scattering during sunsets and sunrises, because they are illuminated with the orange and red light that remains after multiple prior Rayleigh scattering events of the light from the setting/rising sun.
Ash from volcanic eruptions, trapped within the troposphere, tends to mute sunset and sunrise colors, while volcanic ejecta that is instead lofted into the stratosphere (as thin clouds of tiny sulfuric acid droplets), can yield beautiful post-sunset colors 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.
Sometimes just before sunrise or after sunset a green flash can be seen.
Category:Solar phenomena Category:Earth phenomena Category:Symbols Category:Parts of a day
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