Twilight

   

Twilight in Denmark, just after sunset
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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 is the time when the sun is below the horizon, but when no stars are yet visible.
  • Nautical twilight occurs after the centre of the sun is greater than 12 degrees below the horizon and before Astronomical twilight. This is the time when sailors can take reliable star sights of well known stars, using a visible horizon for reference.
  • Astronomical twilight occurs after the centre of the sun is greater than 18 degrees below the horizon and before darkness. Not all stars normally accessible to the naked eye are visible during this time.

In high latitudes in summer time 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

de:Dämmerung it:Crepuscolo ja:薄明 fi:Hämärä


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