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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.
From Wikipedia, the free encyclopedia