Basic Working Principles of a Heliodon

Two Types of Instruments:
We have available in the Heliodon Gallery at CERES two different devices which emulate the dynamic relationships of the sun and earth- which result in the "apparent movement" of the sun across the skyvault. The ring heliodon was designed by Norbert Lechner of Auburn University and was constructed by High Precision Devices, Inc. of Boulder, Colorado. The ring heliodon is most like our experience on Earth wherein the track of sun movement is simulated by the MR lamps located on the circular tubes; these lamps "track" across the skyvault.

A Tilting Earth:
Since the Earth axis is tilted, as it moves around the sun, the northern hemisphere (i.e., the North American continent) tilts toward the sun thus giving us the warmer (summer) season of the year, and as the Earth moves to the opposite position in its orbit around the sun, the northern hemisphere tilts away from the sun, thus giving us the colder (winter) season. Standing on the Earth it is apparent that the result of this tilted-axis-relationship, results in the sun being at differing apparent altitudes above the horizon. In the summertime the sun appears to be very high in the sky, and in the wintertime very low in the sky. Also, the sun appears to rise at different locations along the horizon, not always due east and due west. In fact, the sun only rises due east and due west during the vernal and autumnal equinox, March 21 and September 21. In the wintertime when the sun is at its lowest altitude, it makes its shortest arc across the skyvault; we refer to this as the winter solstice (December 21). Correspondingly, when the sun tracks along the highest and longest arc of the skyvault in the summertime (the summer solstice), on June 21.

A Rotating Earth:
The Earth completely rotates once every 24 hours and given the 360° circumference of its rotation, this means that the apparent position of the sun in the skyvault moves 15 ° every 60 minutes. In the wintertime, at our particular latitude (40° N) along the curve of the Earth's surface, the sun is up during the day for some 8 hours and in the summertime it is up for some 16 hours. At the equinox, day and night are equal in length, 12 hours each.

A Moving Sun; A Moving Model:
In additon to the ring heliodon, CERES also has a platform heliodon. The platform heliodon was designed by Cris Benton and Paul LaBerge of the University of California, Berkeley and was constructed by Ian Melody of Industrial Arts Design in Emeryville, California. In contrast to the mechanical workings of the ring heliodon in which the sun visually tracks across the skyvault, the platform heliodon presents a less intuitive connection to the dynamics of the apparent solar movement. The tradeoff for this is an increased accuracy of the device. In this case, the heliodon itself tilts and the beam of light from the sun (the theater spot located in the ceiling on the second floor) remains fixed. In order to really understand then how the sun would appear, it is necessary to have a camera fixed to the platform itself so that the camera stays in physical relation to the model being examined and thus yields the visual understanding of the movement of the sun across the skyvault. Ironically, this of course is geometrically the more accurate emulation of a rotating Earth and a fixed sun.

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