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   Twilight rays 

 
  Twilight rays Locarno, Switzerland.  Taken by Tony Williams of Liverpool Astronomical Society August 1998. ©Tony Williams, reproduced with permission.
 
Crepuscular derives from the Latin "crepusculum" for twilight. Strictly speaking, only twilight rays should be called crepuscular but in practice the term is used for sunbeams at any time of day. True twilight rays are visible after the sun has set and is up to 4º below the horizon. They are formed from shadows cast by large cloud banks or mountains still in sunlight and up to tens or even several hundred miles away. The rays arch high across the sky and sometimes even form anti-crepuscular rays. Look carefully for them, they are bluish-green against the glowing purple of the twilit sky.