Zeiss planetarium (3D)
About this app
The first projection planetarium of 1923: dome, projector and the 2027 Mars loop in the artificial sky.
With explain mode: an AI tutor walks you through the app, points out what matters in the scene and answers your questions.
How a projection planetarium works
In 1919 Walther Bauersfeld of Carl Zeiss in Jena had the idea of projecting the starry sky from inside onto a fixed dome. On 21 October 1923 Model I showed its sky to the public for the first time at the Deutsches Museum in Munich. A star sphere held 31 partial projectors: light from a 500-watt lamp passed through metal plates with fine holes, and a lens imaged each hole as a star on the dome, about 4,500 stars in all. Separate projectors with gears showed the Sun, Moon and five planets. When the projector turns about the polar axis, the stars rise and set while the dome itself stands still.
What the app shows
Three views: the 16 m dome in cutaway with seating, projector and light rays from the lamp to the stars; the visitor’s view of the dome with constellation names, Milky Way and horizon; and a close-up of the projector with lamp, star plates, lenses and planet cage. The stars come from the Yale Bright Star Catalogue (2,887 stars down to 5.5 mag), the planets from JPL Keplerian elements. A top view of the orbits of Earth and Mars shows how the line of sight from Earth to Mars moves.
What you can try
Set the date and time and start the daily motion: the sky above Munich turns around Polaris. Pick one of the 31 star fields and see which stars its lens casts onto the dome. Play the Mars loop of 2026/27: from January to April 2027 Mars seems to move backwards through Cancer and Leo, with opposition on 19 February 2027. The orbit view shows that Earth is overtaking Mars on the inside track.
Frequently asked questions
- When was the first planetarium shown?
- The first projection planetarium, the Zeiss Model I by Walther Bauersfeld, was shown to the public for the first time on 21 October 1923 at the Deutsches Museum in Munich. It had been tested in a 16-metre dome on the roof of the Zeiss works in Jena. From 7 May 1925 it ran regularly in Munich.
- Why does Mars sometimes move backwards?
- Mars only appears to move backwards. Earth orbits the Sun faster and overtakes Mars on the inside track about every 26 months. While overtaking, the line of sight from Earth to Mars briefly turns back, and Mars draws a loop against the stars. The middle of the loop coincides with opposition.
- How are the stars made in a planetarium?
- In classic star projectors each star is a small hole in a metal or glass plate. A bright lamp shines through it, and a lens images the holes sharply onto the dome. Larger holes make brighter stars. Several such partial projectors together cover the whole sky.
- How accurate are the stars and planets in the app?
- The star positions come from the public-domain Yale Bright Star Catalogue (J2000), the planets from Keplerian elements by E. M. Standish (JPL), valid from 1800 to 2050; the Moon is simplified. The shape and dimensions of the projector and the layout of its 31 fields are schematic.
Subject: Physics experiments (Deutsches Museum)
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