Twilight - Definition and Overview

Twilight in Denmark, just after sunset

Twilight is the time before sunrise and after sunset when sunlight reflected from particles in the upper atmosphere illuminates the lower atmosphere and the surface of the earth.

• Civil twilight begins in the morning when the centre of the sun, as refracted by the atmosphere, is more than 6 degrees below the horizon and ends at sunrise. Evening civil twilight begins at sunset and ends when the centre of the sun, as refracted by the atmosphere, is more than 6 degrees below the horizon. (Typically, the stars are not yet visible.)

• Nautical twilight begins and ends when the centre of the refracted disk of the sun is greater than 12 degrees below the horizon. This is the time when sailors can take reliable star sights of well known stars, using a visible horizon for reference.

• Astronomical twilight begins and ends when the centre of the refracted disk of the sun is greater than 18 degrees below the horizon. Not all stars normally accessible to the naked eye are visible during this time.

(For these definitions, we use an ideal horizon 90 degrees from the zenith.)

In high latitudes in summer the twilight may extend from sunset to sunrise. This phenomenon is known as white night.

External links

• Definition of Twilight (http://aa.usno.navy.mil/faq/docs/RST_defs.html), US Naval Observatory