Astronomy Picture of the Day

                         Astronomy Picture of the Day

    Discover the cosmos! [1] Each day a different image or photograph of our
 fascinating universe is featured, along with a brief explanation written by a
                           professional astronomer.

                              2018 September 23
                                      [2]
                 Equinox: Analemma over the Callanish Stones
                Image Credit & Copyright: Giuseppe Petricca [3]

Explanation: Does the Sun return to the same spot on the sky every day at the
same time? No. A more visual answer to that question is an analemma [4] , a
composite image taken from the same spot at the same time over the course of a
year. The featured analemma was composed from images taken every few days at 4
pm near the village of Callanish [5]  in the Outer Hebrides [6]  in Scotland
[7] , UK [8] . In the foreground are the Callanish Stones [9] , a stone circle
built around 2700 BC during humanity's Bronze Age [10] . It is not known if
the placement of the Callanish Stones [11]  has or had astronomical
significance. The ultimate causes for the figure-8 [12]  shape of this an all
analemmas are the tilt of the Earth axis [13] and the ellipticity [14]  of the
Earth's orbit [15] around the Sun [16] . At the solstice [17] s, the Sun will
appear at the top or bottom of an analemma. Equinox [18] es, however,
correspond to analemma middle points -- not the intersection point. Today at
1:54 am (UT [19] ) is the equinox [20]  ("equal night", when day and night
are equal over all of planet Earth. Many cultures celebrate [21]  a change of
season at an equinox [22] .

               Tomorrow's picture: parts of the sun are missing

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    Authors & editors: Robert Nemiroff [34] (MTU [35] ) & Jerry Bonnell [36]
                                  (UMCP [37] )
          NASA Official:  Phillip Newman Specific rights apply [38] .
              NASA Web Privacy Policy and Important Notices [39]
              A service of: ASD [40]  at NASA [41]  / GSFC [42]
                           & Michigan Tech. U. [43]
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Site notes:
  [1] archivepix.html
  [2] image/1809/CallanishAnalemma_Petricca_1280.jpg
  [3] mailto: gmrpmail @a@t@ gmail .d.o.t. com
  [4] https://en.wikipedia.org/wiki/Analemma
  [5] https://en.wikipedia.org/wiki/Callanish
  [6] https://en.wikipedia.org/wiki/Outer_Hebrides
  [7] https://en.wikipedia.org/wiki/Scotland
  [8] https://en.wikipedia.org/wiki/United_Kingdom
  [9] https://en.wikipedia.org/wiki/Callanish_Stone...
  [10] https://en.wikipedia.org/wiki/Bronze_Age
  [11] https://www.youtube.com/watch?v=mkbEhp8hJN8
  [12] ap160822.html
  [13] https://spaceplace.nasa.gov/seasons/en/
  [14] http://mathworld.wolfram.com/Ellipse.html
  [15] http://bhenvis.nic.in/images/season_climate.j...
  [16] https://solarsystem.nasa.gov/solar-system/sun...
  [17] ap170621.html
  [18] ap170319.html
  [19] https://eclipse.gsfc.nasa.gov/SEhelp/TimeZone...
  [20] https://en.wikipedia.org/wiki/Equinox
  [21] http://mentalfloss.com/article/59049/autumnal...
  [22] ap160320.html
  [23] ap180922.html
  [24] archivepix.html
  [25] lib/apsubmit2015.html
  [26] lib/aptree.html
  [27] https://antwrp.gsfc.nasa.gov/cgi-bin/apod/apo...
  [28] calendar/allyears.html
  [29] /apod.rss
  [30] lib/edlinks.html
  [31] lib/about_apod.html
  [32] http://asterisk.apod.com/discuss_apod.php?dat...
  [33] ap180924.html
  [34] http://www.phy.mtu.edu/faculty/Nemiroff.html
  [35] http://www.phy.mtu.edu/
  [36] https://antwrp.gsfc.nasa.gov/htmltest/jbonnel...
  [37] http://www.astro.umd.edu/
  [38] lib/about_apod.html#srapply
  [39] https://www.nasa.gov/about/highlights/HP_Priv...
  [40] https://astrophysics.gsfc.nasa.gov/
  [41] https://www.nasa.gov/
  [42] https://www.nasa.gov/centers/goddard/
  [43] http://www.mtu.edu/

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