A Sky by Optical Projection

A Sky by Optical Projection

The idea of using optical projection to form a moving artificial sky came first to Walther Bauersfeld of Carl Zeiss Jena shortly after the end of World War I. “The great sphere (the planetarium dome),” he wrote, “shall be fixed; its inner white surface shall serve as the projection surface for many small projectors which shall be placed in the centre of the sphere. The reciprocal positions and motions of the little projectors shall be interconnected by suitable driving gears in such a manner that the little images of the heavenly bodies, thrown upon the fixed hemisphere, shall represent the stars visible to the naked eye, in position and in motion, just as we are accustomed to see them in the natural clear sky”.

In August 1923, after five years of unflagging toil, the new planetarium became a reality. The complex but compact projection instrument first underwent a long series of tests beneath a hemisphere 40 feet in diameter, erected on the roof of the Zeiss Works. So remarkable was its performance that everyone who heard about it was eager to see the Wonder of Jena. Many thousands had an opportunity of seeing it before May, 1925, when it was transferred to the Deutsches Museum, Munich, and where it can still be seen, although only as a Museum exhibit.

Astronomical Power. Illustration: Megan Jorgensen

A second planetarium instrument, similar to the first, was sent to Dusseldorf in 1926, and is now in the Planetarium of the Haagsche Courant, The Hague. The stars formed by each of the two Zeiss planetarium instruments are projected by 31 optical systems, each projecting a pentagonal or hexagonal shaped portion of the artificial sky. Ideally there should be 32 projectors, but one of the fields is cut up by the planetary projector framework at the southern end of the star globe. Some 4,500 stars are projected. They obtain their light from a single 500-watt light bulb mounted at the center of the globe, and before projection consist of small holes pierced in plates of copper foil. Connected to the star globe is a lattice-work cylinder containing mechanisms and projectors for the sun, moon, and five naked-eye planets. These projectors are actuated through suitable gearing by an “annual” drive which can make the planetarium sun circle the ecliptic in periods which range from a few seconds to several minutes. Once the sun’s image is set in motion, the images of the moon and planets go through their own individual and proportionate movements. Finally the star globe and planetary framework can be rotated slowly on a common axis (for precession of the equinoxes), and also on a polar axis (for the daily of diurnal motion).

The first Zeiss projectors had the disadvantage of showing the sky for only one particular latitude. In 1924 Walter Villiger of Zeiss introduced an instrument which could reproduce the sky as seen from any latitude. This “universal” as distinct from “fixed latitude” model had the general shape of a large movable dumb-bell, and after numerous modifications and additions, became the remarkable versatile Zeiss planetarium instrument of modern times.