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 nor completely dark. The sun itself is not actually visible because it is below the horizon. 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.
Twilight is technically defined as the period before sunrise and again after sunset during which there is natural light provided by the upper atmosphere, which does receive direct sunlight and reflects part of it toward the Earth's surface.
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.
