Sky From Wikipedia, the free encyclopedia This article is about Earths sky. For other skies, see extraterrestrial skies. For other uses, see Sky (disambiguation). Page semi-protected Crepuscular rays of light shining through clouds in the sky The sky (or celestial dome) is everything that lies a certain distance[clarification needed] above the surface of the Earth, including the atmosphere and outer space. In the field of astronomy, the sky is also called the celestial sphere. This is an imaginary dome where the sun, stars, planets, and the moon are seen to be traveling. The celestial sphere is conventionally divided into regions called constellations. Usually, the term sky is used from the point of view of the Earths surface; however, the exact meaning of the term can vary. For example, in some cases the sky is defined as only the denser portions of the atmosphere. During daylight, the sky appears to be blue because air scatters blue sunlight more than it scatters red.[1][2][3][4] At night, the sky appears to be a mostly dark surface or region scattered with stars. During the day, the Sun can be seen in the sky, unless obscured by clouds. In the night sky (and to some extent during the day) the moon, planets and stars are visible in the sky. Some of the natural phenomena seen in the sky are clouds, rainbows, and aurorae. Lightning and precipitation can also be seen in the sky during storms. Birds, insects, aircraft, and kites are often considered to fly in the sky. Due to human activities, smog during the day and light pollution during the night are often seen above large cities. Dusk and dawn The Crescent Moon remains visible just moments before Sunrise Main articles: Earths shadow, Green flash and Sky brightness The intensity of the sky varies greatly over the day and the primary cause differs as well. When the sun is well above the horizon, direct scattering of sunlight (Rayleigh scattering) is the overwhelmingly dominant source of light. However, in twilight, the period of time between sunset and sunrise, the situation is more complicated. Green flashes and green rays are optical phenomena that occur shortly after sunset or before sunrise, when a green spot is visible, usually for no more than a second or two, above the sun, or it may resemble a green ray shooting up from the sunset point. Green flashes are a group of phenomena stemming from different causes,[12] most of which occur when there is a temperature inversion (when temperatures increase with altitude rather than the normal decrease in temperature with altitude). Green flashes may be observed from any altitude (even from an aircraft). They usually are seen at an unobstructed horizon, such as over the ocean, but are possible over cloud tops and mountain tops as well. A green flash also may be observed in association with the Moon and bright planets at the horizon, including Venus and Jupiter.[13][14] The Earths shadow is the shadow that the Earth itself casts on its atmosphere, which is often visible from the surface of the Earth, as a dark band in the sky near the horizon. This atmospheric phenomenon can sometimes be seen twice a day, around the times of sunset and sunrise. When the weather conditions and the observers viewing point permit a clear sight of the horizon, the shadow can be seen as a dark blue or greyish-blue band. Assuming the sky is clear, the Earths shadow is visible in the opposite half of the sky to the sunset or sunrise, and is seen right above the horizon as a dark blue band. A related phenomenon is the Belt of Venus or anti-twilight arch (sometimes written as antitwilight arch), a pink band that is visible above the dark blue of the Earths shadow, in the same part of the sky. There is no clear dividing line between the Earths shadow and the Belt of Venus; one colored band shades into the other in the sky.[15][16] Twilight is divided into three segments according to how far the sun is below the horizon in segments of 6°. After sunset the civil twilight sets in, and ends when the sun drops more than 6° below the horizon. This is followed by the nautical twilight, when the
Posted on: Sun, 10 Aug 2014 06:43:17 +0000
Trending Topics
Recently Viewed Topics
© 2015