Twilight is the time between dawn and sunrise, and the time between sunset and dusk. Sunlightscattered 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 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", after the French expression l'heure bleue.

     Twilight is technically defined as the portion of the day during which it is possible to conduct outdoor activities without the aid of artificial light, beginning in the morning and ending in the evening.The period of daylight (between sunrise and sunset) occurs within the period of twilight.

     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.

     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°.

     Note that if the Sun is 8 1/2 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 observed position of the Sun are not to be accounted for.)

SOURCE : http://en.wikipedia.org/wiki/Twilight