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Sky UK
The SKY (or CELESTIAL DOME) is everything that lies above the surface of the Earth , including the atmosphere and outer space . In the field of astronomy , the sky is also called the celestial sphere . This is viewed from Earth's surface as an abstract dome on which the Sun , stars , planets , and Moon appear to be traveling . The celestial sphere is conventionally divided into designated areas called constellations . Usually, the term sky is used informally as the point of view from the Earth's surface; however, the meaning and usage can vary. In some cases, such as in discussing the weather , the sky refers to only the lower, more dense portions of the atmosphere. During daylight , the sky appears to be blue because air scatters more blue sunlight than red. At night, the sky appears to be a mostly dark surface or region spangled with stars. During the day, the Sun can be seen in the sky unless obscured by clouds
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Twilight
TWILIGHT on Earth
Earth
is the illumination of the lower atmosphere when the Sun
Sun
itself is not directly visible because it is below the horizon . Twilight
Twilight
is produced by sunlight scattering in the upper atmosphere, illuminating the lower atmosphere so that Earth\'s surface is neither completely lit nor completely dark. The word twilight is also used to denote the periods of time when this illumination occurs. The farther the Sun
Sun
is below the horizon, the dimmer the twilight (other things such as atmospheric conditions being equal). When the Sun
Sun
reaches 18 degrees below the horizon, the twilight's brightness is nearly zero, and evening twilight becomes nighttime . When the Sun again reaches 18° below the horizon, nighttime becomes morning twilight
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City
A CITY is a large human settlement . Cities generally have extensive systems for housing , transportation , sanitation , utilities , land use , and communication . Their density facilitates interaction between people, government organizations and businesses, sometimes benefiting different parties in the process. Historically, city-dwellers have been a small proportion of humanity overall, but following two centuries of unprecedented and rapid urbanization , roughly half of the world population now lives in cities, which has had profound consequences for global sustainability. Present-day cities usually form the core of larger metropolitan areas and urban areas - creating numerous commuters traveling towards city centers for employment, entertainment, and edification. However, in a world of intensifying globalization , all cities are in different degree also connected globally beyond these regions
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Dawn
DAWN (from an Old English
Old English
verb dagian "to become day") or ASTRONOMICAL DAWN is the time that marks, depending on the specific usage, the beginning of the twilight before sunrise , the period of the pre-sunrise twilight or the time of sunrise. When identified as the beginning of or the period of twilight, it is recognized by the presence of weak sunlight, while the Sun
Sun
itself is still below the horizon . CONTENTS* 1 Types of dawn * 1.1 Astronomical dawn * 1.2 Nautical dawn * 1.3 Civil dawn * 2 Effects of latitude * 2.1 Equator * 2.2 Polar regions * 2.2.1 Example * 3 Mythology and religion * 4 Dawn
Dawn
in art * 5 Literature * 6 See also * 7 References * 8 External links TYPES OF DAWN Main article: Twilight
Twilight
Civil, nautical, and astronomical dawn, when defined as the beginning time of the corresponding twilight
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Atmospheric Optics
ATMOSPHERIC OPTICS deals with how the unique optical properties of the Earth\'s atmosphere cause a wide range of spectacular optical phenomena . The blue color of the sky is a direct result of Rayleigh scattering which redirects higher frequency (blue ) sunlight back into the field of view of the observer . Because blue light is scattered more easily than red light, the sun takes on a reddish hue when it is observed through a thick atmosphere, as during a sunrise or sunset . Additional particulate matter in the sky can scatter different colors at different angles creating colorful glowing skies at dusk and dawn. Scattering
Scattering
off of ice crystals and other particles in the atmosphere are responsible for halos , afterglows , coronas , rays of sunlight , and sun dogs . The variation in these kinds of phenomena is due to different particle sizes and geometries
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Diffuse Sky Radiation
DIFFUSE SKY RADIATION is solar radiation reaching the Earth
Earth
's surface after having been scattered from the direct solar beam by molecules or suspensoids in the atmosphere . It is also called SKYLIGHT, DIFFUSE SKYLIGHT, or SKY RADIATION and is the reason for changes in the color of the sky. Of the total light removed from the direct solar beam by scattering in the atmosphere (approximately 25% of the incident radiation when the sun is high in the sky, depending on the amount of dust and haze in the atmosphere), about two-thirds ultimately reaches the earth as diffuse sky radiation. When the sun is at the zenith in a cloudless sky, with 1361 W/m2 above the atmosphere, direct sunlight is about 1050 W/m2, and total insolation about 1120 W/m2. This implies that under these conditions the diffuse radiation is only about 70 W/m2 out of the original 1361 W/m2
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Light Pollution
LIGHT POLLUTION, also known as PHOTOPOLLUTION, is the presence of anthropogenic light in the night environment. It is exacerbated by excessive, misdirected or obtrusive uses of light, but even carefully used light fundamentally alters natural conditions. As a major side-effect of urbanization, it is blamed for compromising health, disrupting ecosystems and spoiling aesthetic environments
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Smog
SMOG is a type of air pollutant . The word "smog" was coined in the early 20th century as a portmanteau of the words smoke and fog to refer to smoky fog, its opacity, and odour. The word was then intended to refer to what was sometimes known as pea soup fog , a familiar and serious problem in London
London
from the 19th century to the mid 20th century. This kind of visible air pollution is composed of nitrogen oxides, sulphur oxides, ozone, smoke or particulates among others (less visible pollutants include carbon monoxide, CFCs and radioactive sources). Human-made smog is derived from coal emissions, vehicular emissions, industrial emissions, forest and agricultural fires and photochemical reactions of these emissions
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Insect
See text . SYNONYMS * Ectognatha * EntomidaINSECTS or INSECTA (from Latin
Latin
insectum, a calque of Greek ἔντομον , "cut into sections") are by far the largest group of hexapod invertebrates within the arthropod phylum . Definitions and circumscriptions vary; in one approach insects comprise a class within the Phylum
Phylum
Arthropoda. As the term is used here, it is synonymous with ECTOGNATHA. Insects have a chitinous exoskeleton , a three-part body (head , thorax and abdomen ), three pairs of jointed legs , compound eyes and one pair of antennae . They are the most diverse group of animals on the planet, including more than a million described species and representing more than half of all known living organisms . The number of extant species is estimated at between six and ten million, and potentially represent over 90% of the differing animal life forms on Earth
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Aircraft
An AIRCRAFT is a machine that is able to fly by gaining support from the air . It counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil , or in a few cases the downward thrust from jet engines . Common examples of aircraft include airplanes , helicopters , airships (including blimps ), gliders , and hot air balloons . The human activity that surrounds aircraft is called aviation . Crewed aircraft are flown by an onboard pilot , but unmanned aerial vehicles may be remotely controlled or self-controlled by onboard computers. Aircraft
Aircraft
may be classified by different criteria, such as lift type, aircraft propulsion , usage and others
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Kite Flying
A KITE is traditionally a tethered heavier-than-air craft with wing surfaces that react against the air to create lift and drag. A kite consists of wings, tethers, pulleys, and anchors. Kites often have a bridle to guide the face of the kite at the correct angle so the wind can lift it. A kite's wing also may be so designed so a bridle is not needed; when kiting a sailplane for launch, the tether meets the wing at a single point. A kite may have fixed or moving anchors. Untraditionally in technical kiting, a kite consists of tether-set-coupled wing sets; even in technical kiting, though, a wing in the system is still often called the kite. The lift that sustains the kite in flight is generated when air moves around the kite's surface, producing low pressure above and high pressure below the wings. The interaction with the wind also generates horizontal drag along the direction of the wind
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Flight
FLIGHT is the process by which an object moves , through an atmosphere or beyond it, as in the case of spaceflight . This can be achieved by generating aerodynamic lift , propulsive thrust , aerostatically using buoyancy , or by ballistic movement. Many things fly, from natural aviators such as birds , bats and insects to human inventions such as missiles , aircraft such as airplanes , helicopters and balloons , to rockets such as spacecraft . The engineering aspects of flight are the purview of aerospace engineering which is subdivided into aeronautics , the study of vehicles that travel through the air, and astronautics , the study of vehicles that travel through space, and in ballistics , the study of the flight of projectiles
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Rayleigh Scattering
RAYLEIGH SCATTERING (pronounced /ˈreɪli/ RAY-lee ), named after the British physicist Lord Rayleigh
Lord Rayleigh
(John William Strutt), is the (dominantly) elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the radiation. Rayleigh scattering
Rayleigh scattering
does not change the state of material and is, hence, a parametric process . The particles may be individual atoms or molecules. It can occur when light travels through transparent solids and liquids, but is most prominently seen in gases . Rayleigh scattering
Rayleigh scattering
results from the electric polarizability of the particles. The oscillating electric field of a light wave acts on the charges within a particle, causing them to move at the same frequency. The particle therefore becomes a small radiating dipole whose radiation we see as scattered light
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Molecule
A MOLECULE is an electrically neutral group of two or more atoms held together by chemical bonds . Molecules are distinguished from ions by their lack of electrical charge . However, in quantum physics , organic chemistry , and biochemistry , the term molecule is often used less strictly, also being applied to polyatomic ions . In the kinetic theory of gases , the term molecule is often used for any gaseous particle regardless of its composition. According to this definition, noble gas atoms are considered molecules as they are in fact monoatomic molecules. A molecule may be homonuclear , that is, it consists of atoms of one chemical element , as with oxygen (O2); or it may be heteronuclear , a chemical compound composed of more than one element, as with water (H2O). Atoms and complexes connected by non-covalent interactions , such as hydrogen bonds or ionic bonds , are generally not considered single molecules
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Image Gradient
An IMAGE GRADIENT is a directional change in the intensity or color in an image. The gradient of the image is one of the fundamental building blocks in image processing . For example the Canny edge detector uses image gradient for edge detection . In graphics software for digital image editing , the term gradient or color gradient is also used for a gradual blend of color which can be considered as an even gradation from low to high values, as used from white to black in the images to the right. Another name for this is color progression. Mathematically, the gradient of a two-variable function (here the image intensity function) at each image point is a 2D vector with the components given by the derivatives in the horizontal and vertical directions. At each image point, the gradient vector points in the direction of largest possible intensity increase, and the length of the gradient vector corresponds to the rate of change in that direction
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Zenith
The ZENITH is an imaginary point directly "above" a particular location, on the imaginary celestial sphere . "Above" means in the vertical direction opposite to the apparent gravitational force at that location. The opposite direction, i.e. the direction in which gravity pulls, is toward the nadir . The zenith is the "highest" point on the celestial sphere (meaning it is the farthest up from the gravitational force). CONTENTS * 1 Origin * 2 Relevance and use * 3 See also * 4 References ORIGINThe word "zenith" derives from an inaccurate reading of the Arabic expression سمت الرأس (samt ar-ra's) proposed by ancient Persian astronomers, meaning "direction of the head" or "path above the head", by Medieval Latin
Medieval Latin
scribes in the Middle Ages
Middle Ages
(during the 14th century), possibly through Old Spanish
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