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. 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 portion of the day during which it is possible to conduct outdoor activities without the aid of artificial light, beginning in the morning and ending in the evening. The period of daylight (between sunrise and sunset) occurs within the period of twilight.