Twilight is the time before sunrise, called dawn, and the time after sunset, called 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 has not yet come over the horizon (sunrise) or it has passed below the horizon (sunset).

Twilight is specifically defined as the period before or after nighttime during which it is possible to conduct outdoor activities without the aid of artificial light. 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.

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.

Definitions

From a scientific perspective, twilight is defined according to the position of the Sun (its centre) relative to the horizon. There are three established and widely accepted subcategories of twilight: civil twilight (brightest), nautical twilight and astronomical twilight (darkest).

For comparison, the angular diameter of the Sun is 0.5°.

(For these definitions, an ideal horizon 90° from the zenith is used. The altitudes of the sun below the horizon are "true geometric" altitudes, that is, refraction by the atmosphere and other small factors influencing the observed position of the Sun are not to be accounted for.)

Civil twilight

The brightest stars appear during civil twilight, as well as planets, such as Venus, which is known as the 'morning star' and/or 'evening star'. During this period there is enough light from the Sun that artificial sources of light may not be needed to carry on outdoor activities. This concept is sometimes enshrined in laws, like when drivers of automobiles must turn on their headlights, or if the crime of burglary is to be treated as nighttime burglary, which carries stiffer penalties in some jurisdictions. A fixed period (most commonly 30 minutes after sunset or before sunrise) is typically used in such statutes, rather than how many degrees the Sun is below the horizon. Civil twilight can also be described as the limit at which twilight illumination is sufficient, under good weather conditions, for terrestrial objects to be clearly distinguished; at the beginning of morning civil twilight, or end of evening civil twilight, the horizon is clearly defined and the brightest stars are visible under good atmospheric conditions.