Sunrise

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For other uses, see Sunrise (disambiguation).

The terminator in April. As the terminator moves westward across the globe, sunrise occurs north of east, and sunset north of west, except for a thin region near the poles that do not experience sunrise/sunset at all (midnight sun, polar night).

Sunrise in Hollywood, Florida. January 2008

Sunrise in Tehran, Iran. March 2008

An unusual sunrise in New York State.

Sunrise is the time at which the first part of the Sun appears above the horizon in the east. Sunrise should not be confused with dawn, which is the (variously defined) point at which the sky begins to lighten, some time before the sun itself appears, ending twilight. Because atmospheric refraction causes the sun to be seen while it is still below the horizon, both sunrise and sunset are, from one point of view, optical illusions. The sun also appears larger on the horizon, but this is another optical illusion, similar to the moon illusion.

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.

As 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 with the time of year and the latitude of the location from which it is viewed. The precise time of day also varies in local time within a given time zone, determined by each location's precise longitude. Changes in timing of sunrise are driven by the axial tilt of Earth and the planet's movement in its annual orbit around the sun. Some apparent anomalies exist however. In the Northern Hemisphere, the latest sunrise does not occur on the winter solstice around December 21, but rather in early January. Likewise, the earliest sunrise does not fall on the summer solstice around June 21, but occurs earlier in June in the Northern Hemisphere. As one travels farther from the equator, the times of sunrise and sunset change throughout the year. Even on the equator, sunrise and sunset shift several minutes back and forth through the year, along with solar noon. These effects are plotted using 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 precisely due east on the March and September equinoxes for all viewers on Earth, and the duration of day and night are precisely 12 hours on the Equinoxes.

1 Colors
2 See also
3 References
4 External links

[edit] Colors

The red hues of the sky at sunrise and sunset are caused by Rayleigh scattering, the same mechanism that causes the sky to be blue. ^[1] ^[2] ^[3]

As light travels through the atmosphere, some of it is scattered at large angles by small particles. This type of scattering, where the particles are much smaller than the wavelength, is called Rayleigh scattering. Because Rayleigh scattering is much stronger for shorter wavelengths, like blue, the light from a clear sky is blue. Likewise, as the blue component is scattered out of a ray of sunlight, the remaining light becomes yellower. At sunrise or sunset the light has traveled a long distance through the atmosphere leaving only the longest wavelengths like orange and red. The reddened sunlight illuminates clouds and other particles. The combination of orange and red clouds and the blue sky can produce a variety of colors.

Volcanic eruptions release particles into the atmosphere that affect scattering. A number of eruptions in recent times, such as those of Mount Pinatubo in 1991 and Krakatoa in 1883, have been sufficiently large to produce remarkable sunsets and sunrises all over the world.

Sometimes just before sunrise or after sunset a green flash can be seen.

[edit] See also

[edit] References

^ Red Sunset, Green Flash
^ Selected Papers on Scattering in the Atmosphere, edited by Craig Bohren ~SPIE Optical Engineering Press, Bellingham, WA, 1989
^ Science Made Simple