       APOD: 2018 July 13 - Star Trails and the Bracewell Radio Sundial

                         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 July 13
                                      [2]
                 Star Trails and the Bracewell Radio Sundial
          Image Credit & Copyright [3] : Miles Lucas [4]  at NRAO [5]

Explanation: Sundials use [6] the location of a shadow to measure the Earth's
rotation and indicate the time of day. So it's fitting that this sundial, at
the Very Large Array [7] Radio Telescope Observatory in New Mexico,
commemorates the history [8]  of radio astronomy and radio astronomy pioneer
Ronald Bracewell [9] . The radio sundial was constructed using pieces of a
solar mapping radio telescope array that Bracewell orginaly built near the
Stanford University campus. Bracewell's array was used to contribute data to
plan the first Moon landing, its pillars signed by visiting scientists and
radio astronomers, including two Nobel prize winners. As for most sundials the
shadow cast by the central gnomon follows markers that show the solar time of
day, along with solstices and equinoxes. But markers on the radio sundial are
also laid out [10]  according to local sidereal time. They show the position
of the invisible radio shadows of three bright radio sources in Earth's sky,
supernova remnant Cassiopeia A [11] , active galaxy Cygnus A [12] , and active
galaxy Centaurus A [13] . Sidereal time is just star time [14] , the Earth's
rotation as measured with the stars and distant galaxies. That rotation is
reflected in this composited hour-long exposure. Above the Bracewell Radio
Sundial, the stars trace concentric trails around the north celestial pole
[15] .

                    Tomorrow's picture: light-weekend [16]

-------------------------------------------------------------------------------
< [17] | Archive [18] | Submissions [19] | Index [20] | Search [21] | Calendar
  [22] | RSS [23] | Education [24] | About APOD [25] | Discuss [26] | > [27]
-------------------------------------------------------------------------------

    Authors & editors: Robert Nemiroff [28] (MTU [29] ) & Jerry Bonnell [30]
                                  (UMCP [31] )
          NASA Official:  Phillip Newman Specific rights apply [32] .
              NASA Web Privacy Policy and Important Notices [33]
              A service of: ASD [34]  at NASA [35]  / GSFC [36]
                           & Michigan Tech. U. [37]
----------
Site notes:
  [1] archivepix.html
  [2] image/1807/bracewellradiosundial.jpg
  [3] lib/about_apod.html#srapply
  [4] https://mileslucas.myportfolio.com/
  [5] https://public.nrao.edu/
  [6] ap170812.html
  [7] https://public.nrao.edu/
  [8] https://public.nrao.edu/large-arrays-radio-sundial/
  [9] https://aas.org/obituaries/ronald-n-bracewell-1921-2007
  [10] https://public.nrao.edu/wp-content/uploads/2018/03/
Bracewell-Sundial-Brochure_web.pdf
  [11] http://chandra.harvard.edu/photo/2017/casa_life/
  [12] ap150124.html
  [13] ap180712.html
  [14] ap171107.html
  [15] ap131023.html
  [16] ap180714.html
  [17] ap180712.html
  [18] archivepix.html
  [19] lib/apsubmit2015.html
  [20] lib/aptree.html
  [21] https://antwrp.gsfc.nasa.gov/cgi-bin/apod/apod_search
  [22] calendar/allyears.html
  [23] /apod.rss
  [24] lib/edlinks.html
  [25] lib/about_apod.html
  [26] http://asterisk.apod.com/discuss_apod.php?date=180713
  [27] ap180714.html
  [28] http://www.phy.mtu.edu/faculty/Nemiroff.html
  [29] http://www.phy.mtu.edu/
  [30] https://antwrp.gsfc.nasa.gov/htmltest/jbonnell/www/bonnell.html
  [31] http://www.astro.umd.edu/
  [32] lib/about_apod.html#srapply
  [33] https://www.nasa.gov/about/highlights/HP_Privacy.html
  [34] https://astrophysics.gsfc.nasa.gov/
  [35] https://www.nasa.gov/
  [36] https://www.nasa.gov/centers/goddard/
  [37] http://www.mtu.edu/
