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.

                                2020 August 5

                          Picture Rocks Sun Dagger
    Video Credit & Copyright: Martha Schaefer, Brad Schaefer, Jim Stamm;
      Music & License: Awakening (Wojciech Usarewicz), Lone Tree Music

   Explanation: Ancient sun daggers will not hurt you, but they may tell
   you the time.  A sun dagger is a dagger-shaped gap in a shadow created
   by sunlight streaming through a crevice in a nearby rock. Starting over
   a thousand year ago, native people of the American southwest carved
   spiral petroglyphs into rocks that became illuminated by sun daggers in
   different ways as the Sun shifts in the sky. A type of sundial, where
   the end of the sundagger points in the spiral at high noon (for
   example) indicates a time of year, possibly illuminating a solstice or
   equinox.  Sun daggers are thought to have been used by Sun Priests
   during lone vigils with prayers and offerings.  Of the few known, the
   featured video discusses the historic Picture Rocks Sun Dagger near
   Tucson, Arizona, USA, likely created by a Hohokam Sun Priest around
   1000 AD.

                        Tomorrow's picture: 20 and 21
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       Authors & editors: Robert Nemiroff (MTU) & Jerry Bonnell (UMCP)
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