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.
Diffuse sky radiation, is solar radiation reaching the Earth's surface after having been scattered from the direct solar beam by molecules or particulates in the atmosphere. It is also called sky radiation, the determinative process for changing the colors of the sky. It is normally measured on a horizontal surface, thus frequently termed diffuse horizontal irradiance (DHI), often in the unit of watts per square meter (W/m). Approximately 23% of direct incident radiation of total sunlight is removed from the direct solar beam by scattering into the atmosphere; of this amount (of incident radiation) about two-thirds ultimately reaches the earth as photon diffused skylight radiation.
The dominant radiative scattering processes in the atmosphere are Rayleigh scattering and Mie scattering; they are elastic, meaning that a photon of light can be deviated from its path without being absorbed and without changing wavelength.
Hades (/ˈheɪdiːz/; Ancient Greek: ᾍδης, romanized: Hā́idēs, Attic Greek: [háːi̯dεːs], later [háːdeːs]), in the ancient Greek religion and mythology, is the god of the dead and riches and the King of the underworld, with which his name became synonymous. Hades was the eldest son of Cronus and Rhea, although this also made him the last son to be regurgitated by his father. He and his brothers, Zeus and Poseidon, defeated, overthrew, and replaced their father's generation of gods, the Titans, and claimed joint sovereignty over the cosmos. Hades received the underworld, Zeus the sky, and Poseidon the sea, with the solid earth, which was long the domain of Gaia, available to all three concurrently. In artistic depictions, Hades is typically portrayed holding a bidentand wearing his helm with Cerberus, the three-headed guard-dog of the underworld, standing at his side.
Roman-era mythographers eventually equated the Etruscan god Aita,and the Roman gods Dis Pater and Orcus, with Hades, and merged all these figures into Pluto, a Latinisation of Plouton (Ancient Greek: Πλούτων, romanized: Ploútōn), itself a euphemistic title (meaning "the rich one") often given to Hades.
The night sky is the nighttime appearance of celestial objects like stars, planets, and the Moon, which are visible in a clear sky between sunset and sunrise, when the Sun is below the horizon.
Natural light sources in a night sky include moonlight, starlight, and airglow, depending on location and timing. Aurorae light up the skies above the polar circles. Occasionally, a large coronal mass ejection from the Sun or simply high levels of solar wind may extend the phenomenon toward the Equator.
In astronomy and navigation, the celestial sphere is an abstract sphere that has an arbitrarily large radius and is concentric to Earth. All objects in the sky can be conceived as being projected upon the inner surface of the celestial sphere, which may be centered on Earth or the observer. If centered on the observer, half of the sphere would resemble a hemispherical screen over the observing location.
The celestial sphere is a conceptual tool used in spherical astronomy to specify the position of an object in the sky without consideration of its linear distance from the observer. The celestial equator divides the celestial sphere into northern and southern hemispheres.
A sundial is a horological device that tells the time of day (referred to as civil time in modern usage) when direct sunlight shines by the apparent position of the Sun in the sky. In the narrowest sense of the word, it consists of a flat plate (the dial) and a gnomon, which casts a shadow onto the dial. As the Sun appears to move through the sky, the shadow aligns with different hour-lines, which are marked on the dial to indicate the time of day. The style is the time-telling edge of the gnomon, though a single point or nodus may be used. The gnomon casts a broad shadow; the shadow of the style shows the time. The gnomon may be a rod, wire, or elaborately decorated metal casting. The style must be parallel to the axis of the Earth's rotation for the sundial to be accurate throughout the year. The style's angle from horizontal is equal to the sundial's geographical latitude.
The term sundial can refer to any device that uses the Sun's altitude or azimuth (or both) to show the time. Sundials are valued as decorative objects, metaphors, and objects of intrigue and mathematical study.
A sky deity is a deity associated with the sky, and are a common feature of polytheistic religions. The sky often has great religious significance.
The daytime sky deities are typically distinct from the nighttime ones. Stith Thompson's Motif-Index of Folk-Literature reflects this by separating the category of "Sky-god" (A210) from that of "Star-god" (A250). In mythology, nighttime gods are usually known as night deities and gods of stars simply as star gods. Both of these categories are included here since they relate to the sky. Luminary deities are included as well since the sun and moon are located in the sky. Some religions may also have a deity or personification of the day, distinct from the god of the day lit sky, to complement the deity or personification of the night.
In Greek mythology, Uranus (/ˈjʊərənəs/ YOOR-ə-nəs, also /jʊˈreɪnəs/ yoo-RAY-nəs), sometimes written Ouranos (Ancient Greek: Οὐρανός, lit. 'sky', [uːranós]), is the personification of the sky and one of the Greek primordial deities. According to Hesiod, Uranus was the son and husband of Gaia (Earth), with whom he fathered the first generation of Titans. However, no cult addressed directly to Uranus survived into classical times, and Uranus does not appear among the usual themes of Greek painted pottery. Elemental Earth, Sky, and Styx might be joined, however, in solemn invocation in Homeric epic. The translation of his name in Latin is Caelus.