File:Big Tree with Red Sky in the Winter Night.jpg

English: Free picture about the dark, red and yellow night sky that is covered in golden clouds and the frozen frost of the winter. This beautiful tree and sky of the winter season was created for you by the best friend epSos.de and it can be used for free, if you link epSos.de as the original author of the image.

Night — a period during which for a certain point on a surface of a heavenly body the central star is below the horizon line.

Duration "night period" for a certain point depends on its width, an inclination of an axis of rotation of a planet in relation to the plane of its orbit and a corner formed by an axis of rotation and the direction on the central star. The day and night parties of heavenly bodies are divided by terminator.

As axes of rotation of planets of the Solar system aren't perpendicular in relation to the planes of their orbits, duration of nights on them changes during their address round the Sun with year frequency. The identical duration of day and night is observed during equinoxes when the terrestrial axis is perpendicular to an ecliptic; the difference of dlitelnost of day and night is maximum during solstices when an axis of Earth has the maximum inclination in relation to an ecliptic.

In widths above polar circles of planets duration of night can exceed solar days (polar night). Polar night lasts of one days at the width of a polar circle before half a year on a pole.

When lowering Sun for the horizon lower than 18 degrees come astronomically night. When finding the Sun over the horizon — day. The period when the Sun already fell for the horizon, but scattered beams still reach the observer, is called as twilight. If this corner isn't reached, night is called white.

Night sky, it is the term often used to refer to the sky when it is seen during the night. The term is generally associated with the astronomy, with the observation of the stars, the Moon and the planets, which become visible when the Sun hides.

The studies on the night sky have had a place in the ancestral cultures and in the modern ones. In the past, for example, the farmers were observing the night sky as calendar or handlebar to realize the harvests. Many cultures have realized, along the history, representations of the constellations of stars, associating them with legends and mythologies on its deities.

The historical beliefs were based generally on the relations between the celestial bodies or on the events related to the Earth. The "scientific" studies of the night sky and the bodies observed in him have its place in the science of the astronomy.

The visibility of the celestial bodies in the night sky is affected by the light contamination. The presence of the Moon in the sky, has made difficult historically the astronomical observation on having increased the quantity of environmental light, but with the development and the proliferation of the artificial sources of light this problem has increased greatly. There exist special filters that can help to relieve this circumstance, but both for the made fond astronomers and for the professionals, the best solution is to look for places removed from the big cities and other luminous sources.

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