Astronomy Picture of the Day

   [1]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.

                                2026 June 21
     [2]An image of the sky over the Netherlands taken every 15 seconds
    during 2025. Visible are night, day, solstices, equinoxes, moonglow,
    the blue hour, and more. Please see the explanation for more detailed
                                information.

                          Keogram: The Sky in 2025
   Image Credit & [3]License: [4]Cees Bassa ([5]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
   seconds during 2025, an [6]all-sky camera took an image of the sky over
   the [7]Netherlands. Central [8]columns from these images were then
   aligned and combined to create the featured [9]keogram, with January at
   the top, December at the bottom, and the middle of the night running
   vertically just left of center. [10]What do we see? Most obviously, the
   daytime [11]sky is mostly blue, while the nighttime sky is mostly
   black. The twelve [12]light bands crossing the night sky are caused by
   the [13]glow of the [14]Moon. The thinnest [15]part of the black
   hourglass shape occurs during the summer [16]solstice, like [17]today,
   when days are the longest, while the thickest part occurs at the winter
   [18]solstice. Equinoxes can also be located in the keogram, for example
   the northern-spring [19]equinox from one year ago is about
   three-quarters of the way up.

                      Tomorrow's picture: Sun's future
     __________________________________________________________________

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    Authors & editors: [31]Robert Nemiroff ([32]MTU) & [33]Jerry Bonnell
                                 ([34]UMCP)
          NASA Official: Amber Straughn [35]Specific rights apply.
            [36]NASA Web Privacy, [37]Accessibility, [38]Notices;
                A service of: [39]ASD at [40]NASA / [41]GSFC,
                         [42]NASA Science Activation
                           & [43]Michigan Tech. U.

References

   1. https://apod.nasa.gov/apod/archivepix.html
   2. https://apod.nasa.gov/apod/image/2606/Keogram2025_Bassa_960.jpg
   3. https://creativecommons.org/licenses/by-sa/4.0/deed.en
   4. https://orcid.org/0000-0002-1429-9010
   5. https://www.astron.nl/about/
   6. https://apod.nasa.gov/apod/ap011119.html
   7. https://en.wikipedia.org/wiki/Netherlands
   8. https://commons.wikimedia.org/wiki/File:Keogram_explainer.gif
   9. https://victoriaweather.ca/keogram.php#:~:text=What is a keogram
  10. https://i1.sndcdn.com/artworks-000064777927-e3ahj5-t500x500.jpg
  11. https://spaceplace.nasa.gov/blue-sky/en/
  12. https://apod.nasa.gov/apod/ap220301.html
  13. https://theskylive.com/how-bright-is-moon
  14. https://science.nasa.gov/moon/
  15. https://www.astron.nl/~bassa/keogram/
  16. https://svs.gsfc.nasa.gov/14366/
  17. https://en.wikipedia.org/wiki/June_solstice
  18. https://apod.nasa.gov/apod/ap210620.html
  19. https://apod.nasa.gov/apod/ap220320.html
  20. https://apod.nasa.gov/apod/ap260620.html
  21. https://apod.nasa.gov/apod/archivepix.html
  22. https://apod.nasa.gov/apod/lib/apsubmit2015.html
  23. https://apod.nasa.gov/apod/lib/aptree.html
  24. https://antwrp.gsfc.nasa.gov/cgi-bin/apod/apod_search
  25. https://apod.nasa.gov/apod/calendar/allyears.html
  26. https://apod.com/feed.rss
  27. https://apod.nasa.gov/apod/lib/edlinks.html
  28. https://apod.nasa.gov/apod/lib/about_apod.html
  29. https://asterisk.apod.com/discuss_apod.php?date=260621
  30. https://apod.nasa.gov/apod/ap260622.html
  31. https://www.mtu.edu/physics/department/faculty/nemiroff/
  32. https://www.mtu.edu/physics/
  33. https://inspirehep.net/authors/1073087
  34. http://www.astro.umd.edu/
  35. https://apod.nasa.gov/apod/lib/about_apod.html#srapply
  36. https://www.nasa.gov/about/highlights/HP_Privacy.html
  37. https://www.nasa.gov/general/accessibility/
  38. https://www.nasa.gov/privacy/
  39. https://science.gsfc.nasa.gov/astrophysics/
  40. https://www.nasa.gov/
  41. https://www.nasa.gov/centers/goddard/
  42. https://science.nasa.gov/learn/about-science-activation/
  43. http://www.mtu.edu/