                        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.

                                2022 March 21

                               The Sky in 2021
    Image Credit & Copyright: Cees Bassa (Netherlands Institute for Radio
                                 Astronomy)

   Explanation: What if you could see the entire sky -- all at once -- for
   an entire year? That, very nearly, is what is pictured here. Every 15
   minutes during 2021, an all-sky camera took an image of the sky over
   the Netherlands. Central columns from these images were then aligned
   and combined to create the featured keogram, with January at the top,
   December at the bottom, and the middle of the night running vertically
   just left of center. What do we see? Most obviously, the daytime sky is
   mostly blue, while the nighttime sky is mostly black. The twelve light
   bands crossing the night sky are caused by the glow of the Moon. The
   thinnest part of the black hourglass shape occurs during the summer
   solstice when days are the longest, while the thickest part occurs at
   the winter solstice. Yesterday was an equinox -- when night and day
   were equal -- and the northern-spring equinox from one year ago can
   actually be located in the keogram -- about three-quarters of the way
   up.

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                  Tomorrow's picture: a whale of an aurora
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       Authors & editors: Robert Nemiroff (MTU) & Jerry Bonnell (UMCP)
            NASA Official: Phillip Newman Specific rights apply.
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                             & Michigan Tech. U.

