what is twilight

Twilight is a period of time before sunrise and after sunset when the atmosphere is partially illuminated by the sun, being neither totally dark nor completely lit. It is produced by sunlight scattering in the upper atmosphere, which illuminates the lower atmosphere and the Earths surface. There are three categories of twilight that are defined by how far the sun is below the horizon: civil, nautical, and astronomical twilight. The lower the sun is beneath the horizon, the dimmer the twilight, and when the sun reaches 18° 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. Twilight has long been popular with photographers and painters, who often refer to it as the blue hour, after the French expression l'heure bleue, due to its distinctive quality, primarily the absence of shadows and the appearance of objects silhouetted against the lit sky. By analogy with evening twilight, the word twilight is also sometimes used metaphorically to imply an intermediate state that is not clearly defined or a period of decline.