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A planetarium presents shows about astronomy and the night sky.
A planetarium is a theater built primarily for presenting educational and entertaining shows about astronomy and the night sky, or for training in celestial navigation. The plural of "planetarium" is "planetariums" or "planetaria". The term "planetarium" is sometimes used generically to describe other devices which illustrate the solar system, such as a computer simulation or an orrery. The term "planetarian" is used to describe a member of the professional staff of a planetarium.
(Planetarium software refers to a software application that renders a three dimensional image of the sky onto a two dimensional computer screen.)
The most striking feature of most planetaria is their large dome shaped projection screens 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'. These domes can be anything from 3 to 30 m in diameter, accommodating from 1 to 500 people.
Traditionally, planetaria domes were mounted horizontally, matching the natural horizon of the real night sky. However, because that configuration requires highly inclined chairs for comfortable viewing "straight up", increasingly domes are being built tilted from the horizontal by between 5 and 30 degrees to provide greater comfort. Tilted domes tend to create a favoured 'sweet spot' for optimum viewing, centrally about a third of the way up the dome from the lowest point. For this reason, tilted domes generally have seating arranged 'stadium-style' in rows as opposed to the traditional epicentric/circular arrangement of seating common in horizontal domes.
The celestial scenes on the dome can be created using a wide variety of technologies, ranging from precision-engineered 'star balls' that combine optical and electro-mechanical technology, through slide projector, video and digital projector systems to 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 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.
Since the early 1990s, fully featured 3-D digital planetaria have added an extra degree of freedom to a presenter giving a show. They allow simulation of the view from any point in space, not just the earth-bound view with which we are most familiar and to which traditional 'star-ball' planetarium technology is limited. This new virtual reality-capability to travel through the universe provides important educational benefits. It conveys the fact that space has depth vividly, helping audiences to leave behind the ancient misconception that the stars are stuck on the inside of a giant celestial sphere. Instead visitors can understand the true layout of the solar system and beyond. For example, a planetarium can now 'fly' the audience in the direction of one of the familiar constellations such as Orion. This reveals that, in fact, the stars which appear to make up a co-ordinated shape from our earth-bound viewpoint are actually at vastly different distances from Earth and so not really connected at all, except in human imagination and mythology. For audiences with learning styles that are visual or kinesthetic, this can be a particularly memorable demonstration that delivers a learning outcome that would otherwise be hard to achieve.
Archimedes is attributed with possessing a primitive planetarium device that could predict the movements of the Sun, the Moon and the planets. The discovery of the Antikythera mechanism proved that such devices already existed during antiquity. Johannes Campanus (1220-1296) described a planetarium in his "Theorica Planetarum", and included instructions on how to build one.
The first modern planetarium projectors were designed and built by Carl Zeiss in 1924 Germany, and have 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.
In recent years, planetariums, or dome theaters, have broadened their offerings to include wide-screen or "wrap-around" films, all-sky video, and laser shows that combine music with laser-drawn patterns. The newest generation of planetariums such as Evans & Sutherland's Digistar 3 or Sky-Skan's DigitalSky, offer a fully digital projection system, in which a single large projector with a fish eye lens, or a system of digital video or laser video projectors around the edge of the dome, are used to create any scene provided to it from a computer. This gives the operator tremendous flexibility in showing not only the modern night sky as visible from Earth, but any other image they wish (including the night sky as visible from points far distant in space and time).
A portable class of planetariums can be set up for programs at schools, for example, on a temporary basis. Easily transported and quickly erected inflatable structures have been used for this purpose.
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