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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.   
      
                                   2024 April 22   
      A dark mountain ridge is pictured across the foreground at the bottom.   
       Smoke is rising about the ridge, and a close inspection reveals that   
      some of this smoke form rings. The background has a reddish hue, and a   
      crescent Moon is visible on the upper left. Please see the explanation   
                          for more detailed information.   
      
                        Moon and Smoke Rings from Mt. Etna   
                    Image Credit & Copyright: Dario Giannobile   
      
      Explanation: Yes, but can your volcano do this? To the surprise of   
      some, Mt. Etna emits, on occasion, smoke rings. Technically known as   
      vortex rings, the walls of the volcano slightly slow the outside of   
      emitted smoke puffs, causing the inside gas to move faster. A circle of   
      low pressure develops so that the emitted puff of volcanic gas and ash   
      loops around in a ring, a familiar geometric structure that can be   
      surprisingly stable as it rises. Smoke rings are quite rare and need a   
      coincidence of the right geometry of the vent, the right speed of   
      ejected smoke, and the relative calmness of the outside atmosphere. In   
      the featured image taken about two weeks ago from Gangi, Sicily, Italy,   
      multiple volcanic smoke rings are visible. The scene is shaded by the   
      red light of a dawn Sun, while a crescent Moon is visible in the   
      background.   
      
                             Tomorrow's picture: sky X   
        __________________________________________________________________   
      
          Authors & editors: Robert Nemiroff (MTU) & Jerry Bonnell (UMCP)   
               NASA Official: Amber Straughn Specific rights apply.   
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                                & Michigan Tech. U.   
      
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