Star Trails and the Bracewell Radio Sundial


Sundials use 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 Radio Telescope Observatory in New Mexico, commemorates the history of radio astronomy and radio astronomy pioneer Ronald Bracewell.

Star Trails and the Bracewell Radio Sundial

The radio sundial was constructed using pieces of a solar mapping radio telescope array that Bracewell originally 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 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, active galaxy Cygnus A, and active galaxy Centaurus A.

Sidereal time is just star time, 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.

Rings Around the Ring Nebula


There is much more to the familiar Ring Nebula (M57), however, than can be seen through a small telescope. The easily visible central ring is about one light-year across, but this remarkably deep exposure – a collaborative effort combining data from three different large telescopes – explores the looping filaments of glowing gas extending much farther from the nebula’s central star.

Rings Around the Ring Nebula (M57)

This remarkable composite image includes narrowband hydrogen image, visible light emission, and infrared light emission. Of course, in this well-studied example of a planetary nebula, the glowing material does not come from planets. Instead, the gaseous shroud represents outer layers expelled from a dying, sun-like star.

The Ring Nebula is about 2,000 light-years away toward the musical constellation Lyra.

Image Credit: HubbleLarge Binocular TelescopeSubaru TelescopeComposition & Copyright: Robert Gendler