Planetarium, Palais de la Decouverte, Paris

A planetarium (plural planetariums or planetaria) is a theatre built primarily for presenting educational and entertaining shows about astronomy and the night sky, or for training in celestial navigation. A dominant feature of most planetariums is the large dome-shaped projection screen onto which scenes of stars, planets and other celestial objects can be made to appear and move realistically to simulate the complex 'motions of the heavens'. The celestial scenes can be created using a wide variety of technologies, for example precision-engineered 'star balls' that combine optical and electro-mechanical technology, slide projector, video and fulldome projector systems, and lasers. Whatever technologies are used, the objective is normally to link them together to provide an accurate relative motion of the sky. Typical systems can be set to display the sky at any point in time, past or present, and often to show the night sky as it would appear from any point of latitude on Earth.

Planetariums range in size from the Hayden Planetarium's 20-meter dome seating 430 people, to three-meter inflatable portable domes where children sit on the floor. Such portable planetariums serve education programs outside of the permanent installations of museums and science centers.

The term planetarium is sometimes used generically to describe other devices which illustrate the solar system, such as a computer simulation or an orrery. Planetarium software refers to a software application that renders a three dimensional image of the sky onto a two dimensional computer screen. The term planetarian is used to describe a member of the professional staff of a planetarium.

A planetarium projector is a device used to project images of celestial objects onto the dome in a planetarium.

The first modern planetarium projectors were designed and built by the Carl Zeiss Jena company in Germany between 1923 and 1925, and have since grown more complex. Smaller projectors include a set of fixed stars, Sun, Moon, and planets, and various nebulae. Larger machines also include comets and a far greater selection of stars. Additional projectors can be added to show twilight around the outside of the screen (complete with city or country scenes) as well as the Milky Way. Still others add coordinate lines and constellations, photographic slides, laser displays, and other images. The OMNIMAX movie system (now known as IMAX Dome) was originally designed to operate on planetarium screens.

A Zeiss projector is one of a line of planetarium projectors manufactured by the Carl Zeiss Company.

The first modern planetarium projectors were designed and built in 1924 by the Zeiss Works of Jena, Germany in 1924. Zeiss projectors are designed to sit in the middle of a dark, dome-covered room and project an accurate image of the stars and other astronomical objects on the dome. They are generally large, complicated, and imposing machines.

The first Zeiss Mark I projector (the first planetarium projector in the world) was installed in the Deutsches Museum in Munich in August, 1923. It possessed a distinctive appearance, with a single sphere of projection lenses supported above a large, angled "planet cage". Marks II through VI were similar in appearance, using two spheres of star projectors separated along a central axis that contained projectors for the planets. Beginning with Mark VII, the central axis was eliminated and the two spheres were merged into a single, egg-shaped projection unit.

The name "Zeiss projector" is frequently used as a generic term for a star projector, regardless of the manufacturer.^{[citation needed]} Being extremely complex, a large Zeiss projector can cost millions of US dollars.

The Palais de la Découverte is a science museum located in the Grand Palais, in the 8th arrondissement on Avenue Franklin D. Roosevelt, Paris, France. It is open daily except Monday; an admission fee is charged.

The museum was created in 1937 by Jean Baptiste Perrin (awarded the Nobel Prize in Physics, 1926) during an international exhibition on "Arts and techniques in modern life". In 1938 the French government decided to convert the facility into a new museum, which now occupies 25,000 square meters within the west wing of the Grand Palais (Palais d'Antin) built for the Exposition Universelle (1900) to designs by architect Albert-Félix-Théophile Thomas. In January 2010 the museum was merged with the Cité des Sciences et de l'Industrie. The new institution is named universcience.

Today the museum contains permanent exhibits for mathematics, physics, astronomy, chemistry, geology, and biology, featuring interactive experiments with commentaries by lecturers. It includes a Zeiss planetarium with 15-meter dome.