In 1912, Oscar von Miller, founder of the German Museum, proposed the impossible: show visitors the night sky indoors—not a drawing, not a model, but the living movement of celestial bodies, independent of weather, time of day, and city lights. Engineer Walther Bauersfeld from Carl Zeiss Jena responded with a machine that changed the visual language of astronomy and transformed observatory knowledge into spectacle for millions.
Oscar von Miller was no romantic. The founder of the Deutsches Museum in Munich was building a temple of technology—a place where visitors wouldn't read about electricity or steam turbines but see them in action. By 1912, the museum had already assembled a collection of working machines, but for the astronomy department, Miller faced a problem: the sky cannot be brought into a hall. Observatories showed stars through telescopes—one at a time, for one observer. Planispheres and globes were static. Urban light pollution made the real night sky invisible even on the museum's roof.
Miller wanted a hundred people to see simultaneously how the Sun rises above the horizon, how constellations rotate around the celestial pole, how Mars moves among the stars. He wanted a time machine—a device capable of showing the sky of any epoch, any latitude, any hour. The mechanical planetariums of that time—rotating metal structures with balls instead of planets—were too small, too slow, and never created the illusion of a real sky.
The idea hung in the air for ten years. The First World War froze civilian projects, and after the war Miller returned to the task: how to technically create an artificial sky? The solution had to unite optics, mechanics, and architecture—three disciplines that rarely spoke the same language. Miller's first order for an "apparatus for showing the sky" went to Carl Zeiss Jena back in July 1913. The decisive step came in March 1919, when engineer Walther Bauersfeld (by then a 40-year-old veteran of optical calculations) proposed a radically new scheme: not a mechanical structure, but a projection of the sky with light.
Bauersfeld rejected the obvious path. Instead of a giant rotating structure with lamps representing stars, he proposed projecting light onto a dome. The idea seemed simple, but its implementation required solving three problems simultaneously: create a light source for 4,500 stars of varying brightness, synchronize the movement of all projectors so the sky moved as a unified whole, and build a dome that wouldn't distort the image.
The central machine—Zeiss Model I—looked like a single metal sphere only half a meter in diameter, bristling with 31 projection lenses: one hemisphere of sky was projected from one apparatus, so the device weighed about 800 kg at a height of 1.8 m. Inside were metal plates with thousands of tiny holes—one per star. Lamp light passed through the holes, lenses formed a sharp image on the dome. Brightness was coded not by hole size but by rotating discs with slots: a first-magnitude star produced a spot about 23 millimeters on the dome, sixth-magnitude stars—tiny points.
The Sun, Moon, and five planets (Mercury, Venus, Mars, Jupiter, Saturn) required separate projectors capable of independent movement. Bauersfeld placed them in a "planetary cage"—a ring structure around the main sphere. Each projector was controlled by a system of gears imitating real orbits. The Milky Way was projected by a separate device—the diffuse glow of thousands of faint stars that couldn't be shown as individual points.
The entire structure rotated around two axes: one imitated the sky's daily rotation, the other—the Sun's annual movement along the ecliptic. The apparatus was set for the latitude of its installation site—the tilt determined the height of the celestial pole, but this adjustment was made during installation, not during shows. Movement synchronization was ensured by the main motor, auxiliary mechanisms, and hundreds of meters of shafts and gears—the machine resembled a clock mechanism more than a projector. Bauersfeld didn't invent new optics or new lamps; he created an orchestra from existing technologies where each instrument played its part.
October 21, 1923—the prototype was first turned on for the Deutsches Museum commission—the showing took place in Munich, in the museum's unfinished dome about 10 meters in diameter. (The Jena dome—16 meters, concrete over steel mesh on the Zeiss factory roof—received its first viewers later, in summer 1924.) Bauersfeld invited colleagues, Deutsches Museum representatives, and journalists. When the lights went out and the machine came alive, the vault overhead turned into the night sky. Constellations slowly rotated, the Moon rose above the horizon, Venus shimmered in the west. One eyewitness wrote that for a few seconds he forgot he was indoors—the illusion was complete.
The demonstration's success didn't mean readiness for public showing. Bauersfeld spent 1924 fixing defects: motors were noisy, lenses created glare, synchronization failed when speed changed. He moved the prototype to the factory roof and built a new dome—this time from steel mesh covered with a thin layer of concrete. The dome's geodesic structure (which Bauersfeld developed in parallel with the planetarium) was a revolution in itself: light, strong, with no internal supports. That summer began a series of test shows for Jena residents.
Over several months, more than 30,000 people passed under the dome. Lines stretched for several hours. Shows included not only the static sky but time compression: night gave way to day in seconds, a year passed in a minute. Viewers saw the Sun trace an arc across the sky, constellations shift westward, planets loop among the stars. Many came repeatedly to understand what they'd seen the first time—the machine turned out to be not just spectacle but textbook.
Technical reviews were mixed. Astronomers noted the projection was accurate, but star brightness didn't match reality—lamps couldn't reproduce the range from Sirius to the faintest sixth-magnitude stars. Engineers complained about motor noise and maintenance complexity. But the public didn't notice these flaws. For most viewers, this was their first experience observing the full night sky—urban light pollution made real stars invisible even in provincial Jena.
May 7, 1925—the Deutsches Museum opened the first permanent projection planetarium. A dome 10 meters in diameter (about 31 feet) was placed in the museum's west wing, under a specially built hall with a white concrete vault. Zeiss Model I, improved after Jena tests, was installed in the center on a steel platform. The opening ceremony gathered scientists, politicians, and journalists from all over Europe—not so much for technical novelty as for the cultural promise: astronomy no longer required telescopes and clear skies.
The first programs were strictly educational. A lecturer explained celestial motion, showed constellations, demonstrated the difference between the sky at the equator and pole. No music, no visual effects—only voice and the dome's slow rotation. A strict limitation of Model I: the apparatus was rigidly set for Munich's latitude—it couldn't change "sky geography," adjustable latitude only appeared in Model II. The "Year in a Minute" program compressed the motion of the Sun and constellations into a short cycle. Viewers saw not abstraction but concrete physics: why summer days are longer, why Orion is visible at night in winter, why Venus doesn't stray far from the Sun.
The planetarium became a magnet: schools signed up for group sessions months in advance, and lines in Jena for 1924–1926 gathered about 80,000 visitors. Popularity exceeded Miller's expectations—he'd planned an attraction for astronomy enthusiasts and got a mass phenomenon. Some visitors came dozens of times, recording planetary positions and checking them against astronomical tables.
Technically the machine remained imperfect. Star projection was sharp only in the dome's center—at the edge the image blurred due to curvature. The transition between day and night sky looked artificial: lights simply went out, instead of the smooth fading of twilight. The horizon was projected by a separate device, but its line often didn't match the dome's edge, creating a strange visual seam. Despite these flaws, the illusion worked. Many viewers left after the session and looked at the real sky—understanding for the first time what exactly they were seeing.
Success in Munich launched a wave of orders. 1926—a planetarium opened in Jena, in Carl Zeiss's new building. 1927—Vienna: a temporary wooden dome with a linen vault opened on Maria-Theresien-Platz. 1928—Rome, a planetarium opened October 28 in the Aula Ottagona—the octagonal hall of Diocletian's Baths. Each installation required adaptation: domes of different diameters, different climate conditions, different program requirements. Bauersfeld created several modifications of Model I, differing in size and number of projected stars.
1930—Adler Planetarium in Chicago became the first in the Western Hemisphere. American businessman Max Adler saw the planetarium in Munich and decided Chicago needed one. He ordered from Zeiss an improved version with a dome 21 meters in diameter. The opening drew lines stretching several blocks. The American press called the machine a "space theater" and compared it to cinema—both technologies created the illusion of reality through light projection.
1937—Osaka, Japan. The planetarium became part of the city's science museum, the first in Asia. Osaka got a stock Zeiss II with projected ~8,900 stars—no local modifications. By the late 1930s, more than 20 Zeiss planetariums operated in Europe, America, and Asia. The company became a monopolist in this niche—competitors tried to create analogs but couldn't achieve the same optical precision and motion synchronization.
Expansion wasn't smooth. The Second World War destroyed several planetariums, including the first in Munich (damaged by bombing in 1944, restored in the 1950s). The Carl Zeiss factory in Jena ended up in the Soviet occupation zone, leading to the company's split into eastern and western parts. Planetarium production continued in both, but with different modifications. Despite the war, the projection planetarium idea survived—demand for educational tools in the postwar period only grew.
The planetarium was not a project of military propaganda or a vision of the space age. In the 1920s, rocketry existed only in theory: Hermann Oberth published "The Rocket into Planetary Space" in 1923, but there were no practical flights. The idea of crewed space missions seemed as much fantasy as time travel. Bauersfeld wasn't preparing a simulator for future astronauts—he was solving the problem of public education.
When the space age began, planetariums did become infrastructure for teaching navigation, astronomy, and popularizing spaceflight. NASA used planetariums to prepare public opinion for the Apollo program, but also for real training: 62 NASA astronauts passed through the Morehead star hall in North Carolina, honing star navigation. Soviet cosmonauts in the 1960s trained at the Moscow Planetarium—under "secret" classification, disguised as polar pilots. However, the planetarium was never the main simulator: cockpit simulators, centrifuges, and parabolic flights required different technologies.
What Bauersfeld created was a revolution in visual language. Before the planetarium, astronomy spoke the language of tables, formulas, and static maps. The planetarium turned it into cinematic spectacle, where the viewer didn't read about planetary motion but saw it. This shift changed the method of knowledge transmission: from abstraction to direct experience. Millions of people who had never seen a dark sky gained access to astronomical reality—even if through a machine.
The paradox is that the machine created to show the familiar sky prepared the imagination for the unfamiliar. When Yuri Gagarin orbited Earth in 1961, millions of people already had a visual representation of what the sky looks like from space—thanks to planetariums that showed Earth as a planet among others. Bauersfeld didn't plan this, but his machine became a bridge between terrestrial astronomy and the space age, between knowledge and imagination.