Twilight at Baker Beach|250px|thumb

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 nor 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" or "sweet light", after the French expression l'heure bleue. Twilight is technically defined as the period before sunrise and again after sunset during which there is natural light provided by the upper atmosphere, which does receive direct sunlight and reflects part of it toward the Earth's surface.

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

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°.

Note that if the Sun is 8½ degrees below the horizon, it provides the same level of illumination to the surface of the Earth as a full moon directly overhead.

(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 experiential position of the Sun are not to be accounted for.)

Civil twilight

This starts in the morning when the geometric center of the Sun is 6° below the horizon (the point of civil dawn), and ends at sunrise. Evening civil twilight begins at sunset and ends when the center of the Sun reaches 6° below the horizon (the point of civil dusk).

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, for example, when drivers of automobiles must turn on their headlights, when pilots may exercise the rights to fly aircraft, 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.