sky

The sky (or celestial dome) is everything that lies 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 abstract sphere, centered on the Earth, on which the Sun, stars, planets, and the Moon appear to be drifting. The celestial sphere is conventionally divided into designated areas called constellations. Usually, the term sky is used informally as the point of view from the Earth's surface; however, the meaning and usage can vary. An observer on the surface of the Earth can see a small part of the sky, which appears to be a dome, also known as the sky bowl, flatter during the day than at night. In some cases, such as in discussing the weather, the sky refers to only the lower, more dense portions of the atmosphere.
During daytime, the sky appears to be blue because air molecules scatter shorter wavelengths of sunlight more than longer ones (redder light). The night sky appears to be a mostly dark surface or region spangled with stars. The Sun and sometimes the Moon are visible in the daytime sky unless obscured by clouds. At night, the Moon, planets, and stars are similarly visible in the sky. Some of the natural phenomena seen in the sky are clouds, rainbows, and aurorae. Lightning and precipitation are also visible in the sky. Birds, insects, aircraft, and kites fly in the sky. Due to human activities, smog during the day and light pollution during the night are often seen above large cities.

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