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A planetarium (plural planetaria or planetariums) is a theatre built primarily for presenting educational and entertaining shows about astronomy and the night sky, or for training in celestial navigation.[1][2][3]

A dominant feature of most planetaria 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 simulate an accurate relative motion of the sky. Typical systems can be set to simulate the sky at any point in time, past or present, and often to depict the night sky as it would appear from any point of latitude on Earth.

Planetariums range in size from the 37 meter dome in St. Petersburg, Russia (called “Planetarium No 1”) to three-meter inflatable portable domes where attendees sit on the floor. The largest planetarium in the Western Hemisphere is the Jennifer Chalsty Planetarium at Liberty Science Center in New Jersey (27 meters in diameter). The Birla Planetarium in Kolkata, India is the largest by seating capacity (630 seats).[4] Thereafter, the China Science and Technology Museum Planetarium in Beijing, China has the largest seating capacity (442 seats). In North America, the Hayden Planetarium at the American Museum of Natural History in New York City has the greatest number of seats (423).

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 fulldome laser projection.

An increasing number of planetaria are using digital technology to replace the entire system of interlinked projectors traditionally employed around a star ball to address some of their limitations. Digital planetarium manufacturers claim reduced maintenance costs and increased reliability from such systems compared with traditional "star balls" on the grounds that they employ few moving parts and do not generally require synchronisation of movement across the dome between several separate systems. Some planetaria mix both traditional opto-mech

However, the new breed of Optical-Mechanical projectors using fiber-optic technology to display the stars show a much more realistic view of the sky.

An increasing number of planetaria are using digital technology to replace the entire system of interlinked projectors traditionally employed around a star ball to address some of their limitations. Digital planetarium manufacturers claim reduced maintenance costs and increased reliability from such systems compared with traditional "star balls" on the grounds that they employ few moving parts and do not generally require synchronisation of movement across the dome between several separate systems. Some planetaria mix both traditional opto-mechanical projection and digital technologies on the same dome.

In a fully digital planetarium, the dome image is generated by a computer and then projected onto the dome using a variety of technologies including cathode ray tube, LCD, DLP, or laser projectors. Sometimes a single projector mounted near the centre of the dome is employed with a fisheye lens to spread the light over the whole dome surface, while in other configurations several projectors around the horizon of the dome are arranged to blend together seamlessly.

Digital projection systems all work by creating the image of the night sky as a large array of pixels. Generally speaking, the more pixels a system can display, the better the viewing experienc

In a fully digital planetarium, the dome image is generated by a computer and then projected onto the dome using a variety of technologies including cathode ray tube, LCD, DLP, or laser projectors. Sometimes a single projector mounted near the centre of the dome is employed with a fisheye lens to spread the light over the whole dome surface, while in other configurations several projectors around the horizon of the dome are arranged to blend together seamlessly.

Digital projection systems all work by creating the image of the night sky as a large array of pixels. Generally speaking, the more pixels a system can display, the better the viewing experience. While the first generation of digital projectors were unable to generate enough pixels to match the image quality of the best traditional "star ball" projectors, high-end systems now offer a resolution that approaches the limit of human visual acuity.

LCD projectors have fundamental limits on their ability to project true black as well as light, which has tended to limit their use in planetaria. LCOS and modified LCOS projectors have improved on LCD contrast ratios while also eliminating the “screen door” effect of small gaps between LCD pixels. “Dark chip” DLP projectors improve on the standard DLP design and can offer relatively inexpensive solution with bright images, but the black level requires physical baffling of the projectors. As the technology matures and reduces in price, laser projection looks promising for dome projection as it offers bright images, large dynamic range and a very wide color space.

Worldwide, most planetaria provide shows to the general public. Traditionally, shows for these audiences with themes such as "What's in the sky tonight?", or shows which pick up on topical issues such as a religious festival (often the Christmas star) linked to the night sky, have been popular. Pre-recorded and live presentation formats are possible. Live format are preferred by many venues because a live expert presenter can answer on-the-spot questions raised by the audience.

Since the early 1990s, fully featured 3-D digital planetaria have added an extra degree of freedom to a presenter giving a show because they allow simulation of the view from any point in space, not only the earth-bound view which we are most familiar with. This new virtual reality capability to travel through the universe provides important educational benefits because it vividly conveys that space has depth, helping audiences to leave behind the ancient misconception that the stars are stuck on the inside of a giant celestial sphere and instead to understand the true layout of the solar system and beyond. For example, a planetarium can now 'fly' the audience towards one of the familiar constellations such as 3-D digital planetaria have added an extra degree of freedom to a presenter giving a show because they allow simulation of the view from any point in space, not only the earth-bound view which we are most familiar with. This new virtual reality capability to travel through the universe provides important educational benefits because it vividly conveys that space has depth, helping audiences to leave behind the ancient misconception that the stars are stuck on the inside of a giant celestial sphere and instead to understand the true layout of the solar system and beyond. For example, a planetarium can now 'fly' the audience towards one of the familiar constellations such as Orion, revealing that the stars which appear to make up a co-ordinated shape from our earth-bound viewpoint are at vastly different distances from Earth and so not connected, except in human imagination and mythology. For especially visual or spatially aware people, this experience can be more educationally beneficial than other demonstrations.

Music is an important element to fill out the experience of a good planetarium show, often featuring forms of space-themed music, or music from the genres of space music, space rock, or classical music.