What is Twilight?

Twilight is the illumination that is produced by sunlight scattering in the upper atmosphere, illuminating the lower atmosphere when the Sun itself is not directly visible because it is below the horizon, so that the surface of the Earth is neither completely lit nor completely dark. The word "twilight" is also used to denote the periods of time when this illumination occurs.
There are three subcategories of twilight (in order of increasing darkness): Civil, Nautical, and Astronomical. These are discussed below.

Civil Twilight

Civil twilight is the period (morning or evening) when the geometric center of the sun is between the horizon and 6° below the horizon. Civil dawn is the moment when the center of the sun reaches 6° below the horizon prior to sunrise; and civil dusk is the moment when the center of the sun reaches 6° below the horizon after sunset.

The brightest planets are usually visible during civil twilight. Particularly, Venus, the brightest planet as observed from the Earth, is known as the "morning star" or "evening star" due to its visibility during civil twilight. There is enough light from the sun during civil twilight that artificial sources of light may not be needed to carry on outdoor activities.

Civil twilight can be described as the limit at which twilight illumination is sufficient, under clear weather conditions, for terrestrial objects to be clearly distinguished. At the beginning of morning civil twilight or end of evening civil twilight (under good atmospheric conditions with the absence of other illumination), the horizon is clearly defined and the brightest stars are visible.

The military initialisms related to civil twilight are BMCT (begin morning civil twilight, i.e. civil dawn) and EECT (end evening civil twilight, i.e. civil dusk).

Nautical Twilight

Nautical twilight is the period (morning or evening) when the geometric center of the sun is between 6° and 12° below the horizon. Nautical dawn and nautical dusk are defined similarly to civil dawn and civil dusk.

During nautical twilight, the horizon is clearly visible. While general outlines of terrestrial objects may be distinguishable during this time, artificial lighting must be used to see them clearly. The end of this period in the evening, or its beginning in the morning, is also the time at which traces of illumination near the sunset or sunrise point of the horizon are very difficult, if not impossible, to discern (this often being referred to as "first light" before civil dawn and "nightfall" after civil dusk).

Nautical twilight can be described as the limit at which navigation via the horizon at sea is possible. During nautical twilight, sailors can take reliable star sightings of well-known stars, using a visible horizon for reference. At the beginning of morning nautical twilight or at the end of evening nautical twilight (under good atmospheric conditions with the absence of other illumination), general outlines of ground objects may be distinguishable; but detailed outdoor operations are not possible, and the horizon is indistinct.

The military initialisms related to nautical twilight are BMNT (begin morning nautical twilight, i.e. nautical dawn) and EENT (end evening nautical twilight, i.e. nautical dusk). A military unit may treat BMNT and EENT with heightened security, e.g. by "standing to", in which everyone assumes a defensive position. This is partially due to tactics dating back to the French and Indian War (part of the Seven Years' War of 1756–1763), when combatants on both sides would launch attacks at nautical dawn or dusk.

Astronomical Twilight

Astronomical twilight is the period (morning or evening) when the center of the sun is between 12° and 18° below the horizon. Astronomical dawn and astronomical dusk are defined similarly to civil dawn and civil dusk.

From the end of astronomical twilight in the evening to the beginning of astronomical twilight in the morning, the sky (away from urban light pollution) is dark enough for all astronomical observations.

Most casual observers would consider the entire sky fully dark even when astronomical twilight is just beginning in the evening or just ending in the morning, and astronomers can easily make observations of point sources such as stars, but faint diffuse items such as nebulae and galaxies can be properly observed only beyond the limit of astronomical twilight. In some places, especially those with sky glow, astronomical twilight may be almost indistinguishable from night.

Theoretically, the dimmest stars ever visible to the naked eye (those of the sixth magnitude), will become visible at astronomical dusk, and become invisible again at astronomical dawn. However, because of light pollution, some localities, generally those in large cities, may never have the opportunity to view even fourth-magnitude stars, irrespective of the presence of any twilight at all.

The primary source for the information on this page is the Twilight article on Wikipedia, as of 30 June 2013.