This article is about the time of the night. Twilight is the time twilight book pdf read online dawn and sunrise, or between sunset and dusk. Morning: civil, nautical, and astronomical twilight at dawn. The Sun is shown to scale.
Evening: civil, nautical, and astronomical twilight at dusk. Twilight on Earth is the illumination of the lower atmosphere when the Sun itself is not directly visible because it is below the horizon. Twilight is produced by sunlight scattering in the upper atmosphere, illuminating the lower atmosphere so that Earth’s surface is neither completely lit nor completely dark. The word twilight is also used to denote the periods of time when this illumination occurs.
When the Sun reaches 18 degrees below the horizon, the twilight’s brightness is nearly zero, and evening twilight becomes nighttime. Owing to its distinctive quality, primarily the absence of shadows and the appearance of objects silhouetted against the bright sky, twilight has long been popular with photographers, who refer to it as ‘sweet light’, and painters, who refer to it as the blue hour, after the French expression, l’heure bleue. Twilight should not be confused with auroras, which can have a similar appearance in the night sky at high latitudes. By analogy with evening twilight, the word twilight is also sometimes used metaphorically, to imply that something is losing strength and approaching its end.
For example, very old people may be said to be “in the twilight of their lives”. The collateral adjective for twilight is crepuscular which is sometimes applied to insects, fish, and mammals that are most active during that time. Twilight is defined according to the solar elevation angle θs, which is the position of the geometric center of the sun relative to the horizon.
Civil dawn is preceded by morning nautical twilight and civil dusk is followed by evening nautical twilight. Under clear weather conditions, civil twilight approximates the limit at which solar illumination suffices for the human eye to clearly distinguish terrestrial objects. Enough illumination renders artificial sources unnecessary for most outdoor activities. At civil dawn and at civil dusk sunlight clearly defines the horizon while the brightest stars and planets can appear.