Twilight

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 less 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. The brightest stars appear during civil twilight, as well as planets, which may be described as the 'morning star' or 'evening star'. During this period of time there is still enough light from the Sun so that in most cases artificial sources of light are not needed to carry on outdoor activities.

• Nautical twilight is defined as the time when the centre of the refracted disk of the sun is more than 6 degrees degrees below the horizon but less than 12 degrees. This is the time when sailors can take reliable star sights of well known stars, using a visible horizon for reference.

• Astronomical twilight is defined as the time when the centre of the refracted disk of the sun is more than 12 degrees below the horizon but less than 18 degrees. Most casual observers would consider the sky already fully dark towards the end of astronomical twilight, and astronomers can easily make observations of point sources such as stars, but faint diffuse objects such as nebulae and galaxies can only be properly observed after the end of astronomical twilight.

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

Within the polar circles, 24-hour daylight is encountered in summer. In high latitudes outside the polar circles, 24-hour daylight is not seen, but twilight extends from sunset to sunrise, a phenomenon often referred to as 'white nights'. Above 60°N or S, civil twilight lasts all night at midsummer, while above about 55°N or S, nautical twilight lasts all night at midsummer. Astronomical twilight can last all night for several weeks as far from the poles as 50°N or S.

The length of twilight during sunrise and sunset is mostly based on latitude; in the Arctic and Antarctic regions, twilight (if at all) can last for several hours, while at the equator, it goes from day to night in just several minutes. This is due to the fact that at low latitudes the earth spins the fastest and therefore the sun spends less time near the horizon while in polar regions the sun's path in the sky is smaller and thus spends more time near the horizon.

External links

• Definition of Twilight, US Naval Observatory
• Twilight time calculator