Sky

This is an abstract sphere, concentric to the Earth, on which the Sun, Moon, planets, and stars appear to be drifting.

The daytime sky appears blue because air molecules scatter shorter wavelengths of sunlight more than longer ones (redder light).

The Norse term is also the source of the Old English scēo, which shares the same Indo-European base as the classical Latin obscūrus, meaning 'obscure'.

[clarification needed][8] The sky can turn a multitude of colors such as red, orange, purple, and yellow (especially near sunset or sunrise) when the light must travel a much longer path (or optical depth) through the atmosphere.

[9] Red light is also scattered if there is enough air between the source and the observer, causing parts of the sky to change color as the Sun rises or sets.

The effect is not very obvious on clear days, but is very pronounced when clouds cover the line of sight, reducing the blue hue from scattered sunlight.

[10] At higher altitudes, the sky tends toward darker colors since scattering is reduced due to lower air density.

An extreme example is the Moon, where no atmospheric scattering occurs, making the lunar sky black even when the Sun is visible.

[11] Sky luminance distribution models have been recommended by the International Commission on Illumination (CIE) for the design of daylighting schemes.

Green flashes may also be observed at the horizon in association with the Moon and bright planets, including Venus and Jupiter.

No defined line divides Earth's shadow and the Belt of Venus; one colored band fades into the other in the sky.

The term is usually associated with skygazing and astronomy, with reference to views of celestial bodies such as stars, the Moon, and planets that become visible on a clear night after the Sun has set.

Natural light sources in a night sky include moonlight, starlight, and airglow, depending on location and timing.

The presence of the Moon in the night sky has historically hindered astronomical observation by increasing the amount of ambient lighting.

At night, high thin cirrostratus clouds can lead to halos around the Moon, which indicate the approach of a warm front and its associated rain.

[20] Morning fog portends fair conditions and can be associated with a marine layer, an indication of a stable atmosphere.

Within 18 hours of the center's approach, squally weather is common, with sudden increases in wind accompanied by rain showers or thunderstorms.

When the center arrives with a strong tropical cyclone, weather conditions improve and the sun becomes visible as the eye moves overhead.

[24] Flight is the process by which an object moves through or beyond the sky (as in the case of spaceflight), whether by generating aerodynamic lift, propulsive thrust, aerostatically using buoyancy, or by ballistic movement, without any direct mechanical support from the ground.