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                           Astronomy Picture of the Day   
      
       Discover the cosmos! Each day a different image or photograph of our   
         fascinating universe is featured, along with a brief explanation   
                       written by a professional astronomer.   
      
                                 2023 December 20   
      A building is seen from a distance on white snow and with mountains in   
      the background. An ice-crystal filled sky is seen above. Superposed on   
       the night sky are numerous curving whisps -- halos of ice reflecting   
        background moonlight. Please see the explanation for more detailed   
                                   information.   
      
                              Ice Halos over Bavaria   
                     Image Credit & Copyright: Bastian Werner   
      
      Explanation: What's causing those unusual sky arcs? Ice crystals. While   
      crossing a field of fresh snow near F+'ssen, Bavaria, Germany, earlier   
      this month, the photographer noticed that he had entered an ice fog.   
      For suspended water to freeze into an ice fog requires quite cold   
      temperatures, and indeed the air temperature on this day was measured   
      at well below zero. The ice fog reflected light from the Sun setting   
      behind St. Coleman Church. The result was one of the greatest   
      spectacles the photographer has ever seen. First, the spots in the   
      featured picture are not background stars but suspended ice and snow.   
      Next, two prominent ice halos are visible: the 22-degree halo and the   
      46-degree halo. Multiple arcs are also visible, including, from top to   
      bottom, antisolar (subsun), circumzenithal, Parry, tangent, and   
      parhelic (horizontal). Finally, the balloon shaped curve connecting the   
      top arc to the Sun is the rarest of all: it is the heliac arc, created   
      by reflection from the sides of hexagonally shaped ice crystals   
      suspended in a horizontal orientation.   
      
                          Tomorrow's picture: open space   
        __________________________________________________________________   
      
          Authors & editors: Robert Nemiroff (MTU) & Jerry Bonnell (UMCP)   
               NASA Official: Phillip Newman Specific rights apply.   
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                         A service of: ASD at NASA / GSFC,   
                              NASA Science Activation   
                                & Michigan Tech. U.   
      
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