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, centred on the earth, on which the Sun, stars, planets, and Moon appear to be traveling. 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 daylight, the sky appears to be blue because air scatters more blue sunlight than red. At night, the sky appears to be a mostly dark surface or region spangled with stars. During the day, the Sun, and sometimes the Moon, can be seen in the sky unless obscured by clouds. In the night sky the Moon, planets, and stars can be similarly 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. 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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