NASA Photo ID: STS056-21-001       File Name: 10073219.jpg
  Film Type: 35mm                    Date Taken: 04/17/93
  Title: STS-56 Earth observation of a sunset in the southern hemisphere
  Description:
  STS-56 Earth observation shows a sunset in the southern hemisphere, as seen 
  from east to west by the astronaut crewmembers aboard the Earth-orbiting 
  Discovery, Orbiter Vehicle (OV) 103. Classic color changes from reds to
  blues  are caused by white (composite) light passing through density and
  atmospheric  particulate boundaries.  Colors are refracted preferentially
  according to the  diameter of the particles in the visible spectrum with
  longer wavelengths  (reds) refracted the most.  The phenomena can be seen
  here as the reds (0.7  micron) are bent the most while blues (0.4 micron)
  are bent the least.  The  dark substance suspended in the upper
  stratosphere in the left portion of  this scene is of unknown origin.  NASA
  scientists studying the STS-56  photography feel that the most likely
  sources for these substances are  volcanic ash or sulfur dioxide (SO2)
  ejected into the stratosphere by  volcanoes, such as the recent eruptions
  of Mt. Pinatubo (Philippines), Mt  Spurr (Alaska), and Mt. Hudson (Chile).         
  
  Subject terms:
  DISCOVERY (ORBITER)                     
  EARTH ATMOSPHERE                        
  EARTH OBSERVATIONS (FROM SPACE)         
  STRATOSPHERE                            
  STS-56                                  
  SUNSET                                  
  TERMINATOR LINES                        
  

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