Today's Astronomy Picture of the Day features Bracewell's radio
sundial. See
https://apod.nasa.gov/apod/astropix.html
Here is the caption for the picture:
"*Explanation: *Sundials use
<https://apod.nasa.gov/apod/ap170812.html>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
<https://public.nrao.edu/>Radio Telescope Observatory in New Mexico,
commemorates the history
<https://public.nrao.edu/large-arrays-radio-sundial/>of radio astronomy
and radio astronomy pioneer Ronald Bracewell
<https://aas.org/obituaries/ronald-n-bracewell-1921-2007>. 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
<https://public.nrao.edu/wp-content/uploads/2018/03/Bracewell-Sundial-Brochure_web.pdf>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 <http://chandra.harvard.edu/photo/2017/casa_life/>, active
galaxy Cygnus A <https://apod.nasa.gov/apod/ap150124.html>, and active
galaxy Centaurus A <https://apod.nasa.gov/apod/ap180712.html>. Sidereal
time is just star time <https://apod.nasa.gov/apod/ap171107.html>, 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 <https://apod.nasa.gov/apod/ap131023.html>."
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