Twilight is the diffuse illumination of the sky that occurs when the Sun is just below the horizon, creating the periods of dawn and dusk. It is formally divided into three stages based on the Sun's geometric position: civil twilight (0-6° below the horizon, where natural light suffices for most tasks), nautical twilight (6-12°, where the horizon becomes too indistinct for navigation), and astronomical twilight (12-18°, after which true night begins). Because its shadowless, soft light makes objects stand out beautifully, photographers and painters highly prize this period, often calling it the blue hour (l'heure bleue). The term is also used metaphorically to signify decline, as in "the twilight of one's life," and the related adjective crepuscular describes animals most active during this time. Beyond its aesthetic and poetic appeal, twilight has practical legal and regulatory implications, dictating when drivers must switch on headlights, when hunting is allowed, and when aircraft require anti-collision lights.