The sky is an unobstructed view upward from the surface of the Earth. It includes the atmosphere and outer space. It may also be considered a place between the ground and outer space, thus distinct from outer space. In the field of astronomy, the sky is also called the celestial sphere. This is an abstract sphere, concentric to the Earth, on which the Sun, Moon, planets, and stars appear to be drifting. The celestial sphere is conventionally divided into designated areas called constellations. Usually, the term sky informally refers to a perspective 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 resembles a dome appearing flatter during the day than at night. In some cases, such as in discussing the weather, the sky refers to only the lower, denser layers of the atmosphere. The daytime sky appears blue because air molecules scatter shorter wavelengths of sunlight more than longer ones.