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( Twilight)
Twilight is the time between dawn and sunrise, and the time between sunset and dusk. Sunlight scattered in the upper atmosphere illuminates the lower atmosphere, and the surface of the Earth is not completely lit or completely dark. The sun itself is not actually visible because it is below the horizon. Twilight is specifically defined as the period before daytime or after nighttime during which it is possible to conduct outdoor activities without the aid of artificial light.[1] Due to the unusual, romantic quality of the ambient light at this time, twilight has long been popular with photographers and painters, who refer to it as the "blue hour", after the French expression l'heure bleue. The collateral adjective of "twilight" is crepuscular (for daylight it is "diurnal" and for night, "nocturnal"). The term is most frequently encountered when applied to certain species of insects and mammals that are most active during that time. From a scientific perspective, twilight is defined according to the position of the Sun (its centre) relative to the horizon. There are three established and widely accepted subcategories of twilight civil twilight (brightest), nautical twilight and astronomical twilight (darkest).
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