Sky facts for kids

Kids Encyclopedia Facts
The daytime sky with trees.

The sky is what we call the appearance of a hemisphere over our heads. On a clear day it appears blue. The deepness of the blue increases as we look toward the horizon, and up to the point above us.

The sky, which is made up of gas molecules, is blue because of the random scattering of sunlight by the molecules. Rayleigh scattering defines the amount of scattering of light rays. Since all colors of the rainbow create a white light we should see a white sky, but blue light scatters much more than red. That is why the sky appears blue (on a cloudless day).

During the day

Rayleigh sunlight scattering
A greater proportion of blue light scattered by the atmosphere relative to red light.
Twilight subcategories
Civil, nautical, and astronomical twilight. Dusk is the end of evening twilight.
Twilight description full day
Dawn is the beginning of morning twilight.

Except for light that comes directly from the sun, most of the light in the day sky is caused by scattering, which is dominated by a small-particle limit called Rayleigh Scattering. The scattering due to molecule sized particles (as in air) is greater in the direction toward and away from the source of light, than it is in directions perpendicular to the arrival path. Scattering is significant for light at all visible wavelengths but is stronger at the shorter (bluer) end of the visible spectrum, meaning that the scattered light is bluer than its source, the sun. The remaining sunlight, having lost some of its short wavelength components, appears slightly less blue.

Scattering also occurs even more strongly in clouds. Individual water droplets exposed to white light will create a set of colored rings. If a cloud is thick enough, scattering from multiple water droplets will wash out the set of colored rings and create a washed-out white color.

The sky can turn a multitude of colors such as red, orange, purple and yellow (especially near sunset or sunrise) when the light must pass through a much longer path (or optical depth) through the atmosphere. Scattering effects also partially polarize light from the sky and are most pronounced at an angle 90° from the sun. Scattered light from the horizon travels through as much as 38 times the atmosphere as does light from the zenith, causing a blue gradient: vivid at the zenith, and pale near the horizon. Because red light also scatters if there is enough air between the source and the observer causing parts of the sky to change color during a sunset. As the amount of atmosphere nears infinity, the scattered light appears whiter and whiter.

The sun is not the only object that may appear less blue in the atmosphere. Far away clouds or snowy mountaintops may appear yellowish. 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. 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 there is no atmosphere and no scattering, making the sky on the moon black even when the sun is visible.

Sky luminance distribution models have been recommended by the International Commission on Illumination (CIE) for the design of daylighting schemes. Recent developments relate to “all sky models” for modelling sky luminance under weather conditions ranging from clear to overcast.

Dusk and dawn

Moon in Sunrise Sky 2
The crescent moon remains visible just moments before sunrise

The intensity of the sky varies greatly over the day, and the primary cause of that intensity 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 night and between night 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 above the sun, usually for no more than a second or two, or it may resemble a green ray shooting up from the sunset point. Green flashes are a group of phenomena that stem from different causes, most of which occur when there is a temperature inversion (when the temperature increases with altitude rather than the normal decrease in temperature with altitude). Green flashes may be observed from any altitude (even from an aircraft). They are usually seen at an unobstructed horizon, such as over the ocean, but are also seen over cloud tops and mountain tops. Green flashes may also be observed at the horizon in association with the Moon and bright planets, including Venus and Jupiter.

The Earth's shadow is the shadow that the Earth casts on its atmosphere. This atmospheric phenomenon is sometimes seen twice a day, around the times of sunset and sunrise. When the weather conditions and the observer's 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 Earth's shadow is visible in the half of the sky opposite to the sunset or sunrise, and is seen as a dark blue band right above the horizon. A related phenomenon is the "Belt of Venus" or "anti-twilight arch", a pink band that is visible above the dark blue band of the Earth's shadow in the same part of the sky. There is no clear dividing line between the Earth's shadow and the Belt of Venus: one colored band shades into the other in the sky.

Twilight is divided into three segments according to how far the sun is below the horizon, measured in segments of 6°. After sunset the civil twilight sets in; it ends when the sun drops more than 6° below the horizon. This is followed by the nautical twilight, when the sun is 6° and 12° below the horizon (heights of between −6° and −12°), after which comes the astronomical twilight, defined as the period from −12° to −18°. When the sun drops more than 18° below the horizon, the sky generally attains its minimum brightness.

Several sources can be identified as the source of the intrinsic brightness of the sky, namely airglow, indirect scattering of sunlight, scattering of starlight, and artificial light pollution.

During the night

Polarlicht 2
Aurora borealis over Bear Lake, Alaska
Deathvalleysky nps big
The Milky Way can be seen as a large band across the night sky, and is distorted into an arch in this 360° panorama.

The term night sky refers to the sky as seen at night. 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 fact that the sky is not completely dark at night can be easily observed. Were the sky (in the absence of moon and city lights) absolutely dark, one would not be able to see the silhouette of an object against the sky.

The night sky and studies of it have a historical place in both ancient and modern cultures. In the past, for instance, farmers have used the state of the night sky as a calendar to determine when to plant crops. The ancient belief in astrology is generally based on the belief that relationships between heavenly bodies influence or convey information about events on Earth. The scientific study of the night sky and bodies observed within it, meanwhile, takes place in the science of astronomy.

Within visible-light astronomy, the visibility of celestial objects in the night sky is affected by light pollution. The presence of the Moon in the night sky has historically hindered astronomical observation by increasing the amount of ambient lighting. With the advent of artificial light sources, however, light pollution has been a growing problem for viewing the night sky. Special filters and modifications to light fixtures can help to alleviate this problem, but for the best views, both professional and amateur optical astronomers seek viewing sites located far from major urban areas.

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Sky Facts for Kids. Kiddle Encyclopedia.