An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

One of the twilight phases when the Sun's center lies between 12 and 18 degrees below the horizon. Astronomical twilight is followed or preceded by → nautical twilight. Most stars and other celestial objects can be seen during this phase. However, some of the fainter stars and galaxies may not be observable as long as the Sun is less than 18 degrees below the horizon. See also → civil twilight.

→ astronomical; → twilight.

The time between sunset or sunrise and the moment when the Sun's center lies 6° below the horizon. It is followed or preceded by → nautical twilight. See also → astronomical twilight. In the morning, this twilight phase ends at sunrise. In the evening it begins at sunset. Civil twilight is the brightest of the three twilight phases. As the Earth's atmosphere scatters and reflects much of the Sun's rays, artificial lighting is generally not required in clear weather conditions to carry out most outdoor activities. Only the brightest stars and planets, like Venus and Jupiter, can be seen with the naked eye.

→ civil; → twilight.

One of the three twilight phases which is the period before sunrise and after sunset when the center of the Sun's disk is between 6° and 12° below the horizon. This twilight phase is followed or preceded by → civil twilight. See also → astronomical twilight. In clear weather conditions, the horizon is faintly visible during this phase. Many of the brighter stars can also be seen, making it possible to use the position of the stars in relation to the horizon to navigate at sea. This is why it is called nautical twilight.

→ nautical astronomy; → twilight.

The diffused light from the sky when the Sun is below the → horizon, either from daybreak to → sunrise or, more commonly, from → sunset to → dusk or nightfall. There are three types of twilight: → astronomical twilight, → nautical twilight, and → civil twilight. They are divided on the basis of the → solar depression angle.

M.E., cognate with Du. tweelicht, Ger. zwielicht, from twi- a combining form meaning "two, twice," but it appears to refer to "half" light, rather than the fact that twilight occurs twice a day + → light.

Nimtâb, from nim, nimé, → half, + tâb "light," present stem of tâbidan "to shine," → luminosity.