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Landers, California at astronomical dusk, 20 second exposure.

Dusk occurs at the darkest stage of twilight, or at the very end of astronomical twilight after sunset and just before night.^[1] Pre-dusk, during early to intermediate stages of twilight, there may be enough light in the sky under clear conditions to read outdoors without artificial illumination, but at the end of civil twilight, when Earth rotates to a point at which the center of the Sun is 6° below the local horizon, artificial illumination is required to read outside.^[2] The term dusk usually refers to astronomical dusk, or the darkest part of twilight before night begins.

Technical definitions

Civil, nautical, and astronomical twilight.^[3] Dusk is the darkest part of evening twilight.

The time of dusk is the moment at the very end of astronomical twilight, just before the minimum brightness of the night sky sets in, or may be thought of as the darkest part of evening twilight.^[4]
But technically, the three stages of dusk are as follows.

• At civil dusk, the center of the Sun's disc goes 6° below the horizon in the evening. It marks the end of civil twilight, which begins at sunset. At this time objects are still distinguishable and depending on weather conditions some stars and planets may start to become visible to the naked eye. The sky has many colors at this time, such as orange and red. Beyond this point artificial light may be needed to carry out outdoor activities, depending on atmospheric conditions and location.


• At nautical dusk, the Sun apparently moves to 12° below the horizon in the evening. It marks the end of nautical twilight, which begins at civil dusk. At this time, objects are less distinguishable, and stars and planets appear to brighten.


• At astronomical dusk, the Sun's position is 18° below the horizon in the evening. It marks the end of astronomical twilight, which begins at nautical dusk. After this time the Sun no longer illuminates the sky, and thus no longer interferes with astronomical observations.^[2][5]

Gallery

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  Seattle, Washington during Nautical twilight
  

  
  

  
  


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  Civil twilight in Porto Covo, west coast of Portugal
  

  
  

  
  


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  Astronomical twilight as seen from a plane window
  

  
  

  
  


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  Magere Brug Amsterdam - Astronomical twilight
  

  
  

  
  


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  View from Ibirapuera Park, São Paulo, Brazil, at Civil twilight
  

  
  

  
  


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  Trees silhouette dusk sky
  

  
  

  
  


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  Civil twilight in Albi, France
  

  
  

  
  


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  Nautical twilight in Vancouver, British Columbia, Canada
  

  
  

  
  

See also

• Dawn


• Sunrise


• Sunset


• Twilight

References