As observed from the Earth (see apparent magnitude), sky-gazers know Venus, the brightest planet, as the 'morning star' or 'evening star' because they can see it during civil twilight. At civil dawn and at civil dusk sunlight clearly defines the horizon while the brightest stars and planets can appear. Enough illumination renders artificial sources unnecessary for most outdoor activities. Under clear weather conditions, civil twilight approximates the limit at which solar illumination suffices for the human eye to clearly distinguish terrestrial objects. The collateral adjective for twilight is crepuscular, which may be used to describe the behavior of animals that are most active during this period.Ĭivil twilight in a small town in the Mojave Desert For example, very old people may be said to be 'in the twilight of their lives'. Owing to its distinctive quality, primarily the absence of shadows and the appearance of objects silhouetted against the lit sky, 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.īy analogy with evening twilight, the word twilight is also sometimes used metaphorically, to imply that something is losing strength and approaching its end. When the Sun again reaches 18° below the horizon, nighttime becomes morning twilight. When the Sun reaches 18° below the horizon, the twilight's brightness is nearly zero, and evening twilight becomes nighttime. The lower the Sun is beneath the horizon, the dimmer the twilight (other factors such as atmospheric conditions being equal).

The word twilight can also refer to the periods of time when this illumination occurs. Twilight is light produced by sunlight scattering in the upper atmosphere, when the Sun is below the horizon, which illuminates the lower atmosphere and the Earth's surface. Evening twilight: civil, nautical, and astronomical stages at dusk.

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