Skyglow in the context of "Zodiacal light"

Light pollution refers to artificial lighting that alters the natural patterns of light and dark in ecosystems. In a descriptive sense, light pollution occurs when artificial light is used in a poor or excessive way, especially at night, causing negative effects in the environment and the view of the night sky. Light pollution can be understood not only as a phenomenon resulting from a specific source or kind of pollution, but also as a contributor to the wider, collective impact of various sources of pollution.

Although this type of pollution can exist throughout the day, its effects are magnified during the night with the contrast of the sky's darkness. It has been estimated that 83% of the world's people live under light-polluted skies and that 23% of the world's land area is affected by skyglow.The area affected by artificial illumination continues to increase. A major side effect of urbanization, light pollution is blamed for compromising health, disrupting ecosystems, and spoiling aesthetic environments. Studies show that urban areas are more at risk. Globally, it has increased by at least 49% from 1992 to 2017.

Aircraft camouflage is the use of camouflage on military aircraft to make them more difficult to see, whether on the ground or in the air. Given the possible backgrounds and lighting conditions, no single scheme works in every situation. A common approach has been a form of countershading, the aircraft being painted in a disruptive pattern of ground colours such as green and brown above, sky colours below. For faster and higher-flying aircraft, sky colours have sometimes been used all over, while helicopters and fixed-wing aircraft used close to the ground are often painted entirely in ground camouflage. Aircraft flying by night have often been painted black, but this actually made them appear darker than the night sky, leading to paler night camouflage schemes. There are trade-offs between camouflage and aircraft recognition markings, and between camouflage and weight. Accordingly, visible light camouflage has been dispensed with when air superiority was not threatened or when no significant aerial opposition was anticipated.

Aircraft were first camouflaged during World War I; aircraft camouflage has been widely employed since then. In World War II, disruptive camouflage became widespread for fighters and bombers, sometimes combined with countershading. Some air forces such as the German Luftwaffe varied their paint schemes to suit differing flight conditions such as the skyglow over German cities, or the sands of the Mediterranean front.

DarkSky International, formerly the International Dark-Sky Association (IDA), is a United States–based nonprofit organization incorporated in 1988 by founders David Crawford, a professional astronomer, and Tim Hunter, a physician and amateur astronomer. The mission of DarkSky is "to preserve and protect the night time environment and our heritage of dark skies through quality outdoor lighting".

Light pollution is the result of outdoor lighting that is not properly shielded, allowing light shine into the eyes and night sky. Direct light that shines into the eyes is called glare, and light directed into the night sky above the horizon causes skyglow. Lighting can also cause light trespass when it enters areas where unwanted (e.g. a neighbor's yard and windows). DarkSky is the first and currently the largest organization in the dark-sky movement.

The dark-sky movement is a campaign to reduce light pollution. The advantages of reducing light pollution include an increased number of stars visible at night, reducing the effects of electric lighting on the environment, improving the well-being, health and safety of people and wildlife, and cutting down on energy usage. Earth Hour and International Dark-Sky Week are two examples of such efforts.

The movement started with professional and amateur astronomers alarmed that nocturnal skyglow from urban areas was blotting out the sight of stars. For example, the world-famous Palomar Observatory in California is threatened by sky-glow from the nearby city of Escondido and local businesses. For similar reasons, astronomers in Arizona helped push the governor there to veto a bill in 2012 which would have lifted a ban on illuminated billboards.