The 17th-century optical planetarium that projected a royalist night sky

Technology · Published 1 November 2026 Swipe more like this →

In short: Before modern technology allowed us to map the heavens, humans built physical models of the cosmos. One such device, created in 1699, didn't just show the stars—it used light and copper to project them onto a room, turning an astronomical tool into a theatrical experience of the universe.

The celestial globe serves as a spherical map of the visible heavens, designed to put objects like constellations and astronomical circles into perspective. Unlike terrestrial globes that map geography, these models place the Earth at an imaginary center. To function correctly, they are mounted in harnesses that allow them to rotate and tilt to different latitudes, mimicking the movement of the sky as seen from various points on our planet.

A notable example is a celestial globe built near the turn of the 18th century by Erhardt Weigel, a professor of Astronomy at the University of Jena. This specific model functions as an early optical planetarium: a lamp placed in its hollow center projects light through small holes punched into an embossed copper sphere. By looking through four larger apertures, a viewer sees the stars as points of light against a dark background, accurately configured to represent the night sky.

However, Weigel’s globe contains a unique political twist. Rather than using standard pagan figures for constellations, he replaced them with symbols representing European royal families. In his design, for instance, the constellation Orion was transformed into a double-headed Austrian eagle. This highlights how these instruments served not just as scientific tools, but as physical illustrations of the specific beliefs and power structures of their era.

The construction of such globes also presents a geometric challenge known as 'handedness.' Because a globe is an orthographic projection viewed from the outside, while our view of the sky is a gnomonic projection from the inside, stars can appear mirrored. To solve this, some makers produce mirror-image globes intended to be viewed in a reflection, or use transparent surfaces that unfortunately introduce significant visual distortions.

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