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.

                                2021 April 18

                       Rainbow Airglow over the Azores
     Image Credit & Copyright: Miguel Claro (TWAN); Rollover Annotation:
                                Judy Schmidt

   Explanation: Why would the sky glow like a giant repeating rainbow?
   Airglow. Now air glows all of the time, but it is usually hard to see.
   A disturbance however -- like an approaching storm -- may cause
   noticeable rippling in the Earth's atmosphere. These gravity waves are
   oscillations in air analogous to those created when a rock is thrown in
   calm water. The long-duration exposure nearly along the vertical walls
   of airglow likely made the undulating structure particularly visible.
   OK, but where do the colors originate? The deep red glow likely
   originates from OH molecules about 87-kilometers high, excited by
   ultraviolet light from the Sun. The orange and green airglow is likely
   caused by sodium and oxygen atoms slightly higher up. The featured
   image was captured during a climb up Mount Pico in the Azores of
   Portugal. Ground lights originate from the island of Faial in the
   Atlantic Ocean. A spectacular sky is visible through this banded
   airglow, with the central band of our Milky Way Galaxy running up the
   image center, and M31, the Andromeda Galaxy, visible near the top left.

                Explore Your Universe: Random APOD Generator
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