National Radio Astronomy Observatory

The Very Large Array’s Radio Sundial

Signatures from Radio Astronomy notables adorn the pillars at the VLA Radio Sundial

In 1955, Australian radio astronomer Ron Bracewell joined the faculty of Stanford University and established a radio astronomy observatory on the campus. Constructing an array of 32 10-foot-diameter antennas – each of which was mounted on a concrete pillar, and dubbed the Stanford “Cross.”  The radio array was used to make daily maps of the sun from June 1962 to August 1973 and to study, among other things, the refraction phenomena in the high solar corona. Over the course of 20 years, Bracewell invited visiting astronomers to chisel their names onto these pillars, amassing 220 signatures from scientists and Noble Prize winners — sort of a who’s who in the radio astronomy world.  After the telescope was decommissioned in the early 1980’s, preservationists worked diligently to recover parts of the array. In 2012, the NRAO was able to secure 10 of remaining pillars and each one was transported to the Karl G. Jansky Very Large Array in New Mexico, where a team of radio astronomers fashioned a radio sundial.

The idea of a radio sundial seems oxymoronic.  Radio waves are invisible to the human eye and only a small portion of the electromagnetic spectrum is visible to us. When we image radio waves through radio telescopes, we have to use quite a bit of complicated math to render the images into something that we’re used to seeing.  So there was a bit of a conundrum in trying to translate this conventional concept of a sundial — that outlines the sun’s shadow on a plate aligned with the hours of the day — to one that instructs visitors to imagine the approximate positions of three discrete radio sources.

Using the decommissioned pillars from the Stanford radio telescope, W.T. Sullivan III, designed a sundial that functions both as an optical indicator of time and one in which local sidereal time (star time) can be calculated to approximate the location of three important radio sources: Centaurus A, Cygnus A, and Cassiopeia A. Unlike any other sundial in the world, it also allows visitors to find the approximate position of two distant galaxies and the remains of an exploded star in the Milky Way. This sundial provides an imaginative window into our invisible universe but also serves as an important memorial to Ron Bracewell who developed many of the mathematical techniques that are used in radio imaging processes today.