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A planetarium 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.
- The plural of planetarium can be either 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.
Planetariums have become well-nigh ubiquitous, with some privately owned. A rough estimate is that in the United States there is one planetarium per 100,000 population, ranging 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.
See also the Timeline of planetariums for specific dates and events in the historical influences on and development of planetariums.
Archimedes is attributed with possessing a primitive planetarium device that could predict the movements of the Sun and 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. These devices would today usually be referred to as orreries (named for the Earl of Orrery, a place in Ireland: an 18th century Earl of Orrery had one built). In fact, many planetariums today have what are called projection orreries, which project onto the dome a Sun with planets (usually limited to Mercury up to Saturn) going around it in something close to their correct relative periods.
The small size of typical 18th century orreries limited their impact, and towards the end of that century a number of educators attempted some larger scale simulations of the heavens. The efforts of Adam Walker (1730-1821) and his sons are noteworthy in their attempts to fuse theatrical illusions with educational aspirations. Walker's Eidouranion was the heart of his public lectures or theatrical presentations. Walker's son describes this "Elaborate Machine" as "twenty feet high, and twenty-seven in diameter: it stands vertically before the spectators, and its globes are so large, that they are distinctly seen in the most distant parts of the Theatre. Every Planet and Satellite seems suspended in space, without any support; performing their annual and diurnal revolutions without any apparent cause". Other lecturers promoted their own devices: R E Lloyd advertised his Dioastrodoxon, or Grand Transparent Orrery, and by 1825 William Kitchener was offering his Ouranologia, which was 42 feet in diameter. These devices most probably sacrificed astronomical accuracy for crowd-pleasing spectacle and sensational and awe-provoking imagery.
The oldest, still working planetarium can be found in the Dutch town Franeker. It was build by Eise Eisinga (1744-1828) in the livingroom of his house. It took Eisinga seven years to build his planetarium, which was completed in 1781.
In 1905 Oskar von Miller (1855-1934) of the Deutsches Museum in Munich commissioned updated versions of a geared orrery and planetarium from M Sendtner, and later worked with Franz Meyer, chief engineer at the Carl Zeiss optical works in Jena, on the largest mechanical planetarium ever constructed, capable of displaying both heliocentric and geocentric motion. This was displayed at the Deutsches Museum in 1924, construction work having been interrupted by the war. The planets travelled along overhead rails, powered by electric motors: the orbit of Saturn was 11.25 m in diameter. 180 stars were projected onto the wall by electric bulbs.
While this was being constructed, von Miller was also working at the Zeiss factory with German astronomer Max Wolf, former director of the Baden Observatory in Heidelberg, on a new and novel design, inspired by Wallace W Atwood's work at the Chicago Academy of Sciences and by the ideas of Walther Bauersfeld at Zeiss. The result was a planetarium design which would generate all the necessary movements of the stars and planets inside the optical projector, and would be mounted centrally in a room, projecting images onto the white surface of a hemisphere. In August 1923, the first Zeiss planetarium projected images of the night sky onto the white plaster lining of a 16 m hemispherical concrete dome, erected on the roof of the Zeiss works.
A great boost to the popularity of the planetarium worldwide was provided by the Space Race of the 1950s and 60s when fears that the United States of America might miss out on the opportunities of the new frontier in space stimulated a massive program to install over 1,200 planetariums in U.S. high schools.
 After WWII
After the war when Germany was split, this split the Zeiss firm. Part remained in its traditional headquarters at Jena, now in East Germany, and part migrated to West Germany. The designer of the first planetariums for Zeiss, Walther Bauersfeld, remained in Jena until his death in 1959.
The West German firm resumed making large planetariums in 1954, and the East German firm started making small planetariums a few years later. Meanwhile, the lack of planetarium manufacturers had led to several attempts at construction of unique models, such as one built by the California Academy of Sciences in Golden Gate Park, San Francisco, which was unique in being the first (and for a very long time only) planetarium to project the planet Uranus. Most planetariums ignore Uranus as being at best marginally visible to the naked eye.
Armand Spitz recognized that there was a viable market for small inexpensive planetariums. His first model, the Spitz A, was designed to project stars from a dodecahedron, thus reducing machining expenses in creating a globe. Planets were not mechanized, but could be shifted by hand. Several models followed with various upgraded capabilities, until the A3P, which projected well over a thousand stars, had motorized motions for latitude change, daily motion, and annual motion for Sun, Moon (including phases), and planets. This model was installed in hundreds of high schools, colleges, and even small museums from 1964 to the 1980s.
Japan entered the planetarium manufacturing business in the 1960s, with Goto and Minolta both successfully marketing a number of different models. Goto was particularly successful when the Japanese Ministry of Education put one of their smallest models, the E-3 or E-5 (the numbers refer to the metric diameter of the dome) in every elementary school in Japan.
Phillip Stern, as former lecturer at New York City's Hayden Planetarium, had the idea of creating a small planetarium which could be programmed. His Apollo model was introduced in 1967 with a plastic program board, recorded lecture, and film strip. Unable to pay for this himself, Stern became the head of the planetarium division of Viewlex, a mid-size audio-visual firm on Long Island. About thirty canned programs were created for various grade levels and the public, while operators could create their own or run the planetarium live. Purchasers of the Apollo were given their choice of two canned shows, and could purchase more. A few hundred were sold, but in the late 1970s Viewlex went bankrupt for reasons unrelated to the planetarium business.
During the 1970s, the OmniMax movie system (now known as IMAX Dome) was conceived to operate on planetarium screens. More recently, some planetariums have re-branded themselves as dome theaters, with broader offerings including wide-screen or "wraparound" films, fulldome video, and laser shows that combine music with laser-drawn patterns.
StarLab in Massachusetts offered the first easily portable planetarium in 1977 which projected stars, constellation figures from many mythologies, celestial coordinate systems, and much else, from removable cylinders. (Viewlex had a portable version of the Apollo earlier.)
When Germany reunified in 1989, the two Zeiss firms did likewise, and expanded their offerings to cover many different size domes.
 Computerized planetariums
The newest generation of planetariums such as Evans & Sutherland's Digistar 3, RSA Cosmos's InSpace System or Sky-Skan's DigitalSky, offer a fully digital projection system, using fulldome video technology. This gives the operator great 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 new generation of home planetariums was released in Japan by Takayuki Ohira in cooperation with Sega. Ohira is worldwide known as a mastermind for building portable planetariums used at exhibitions and events such as the Aichi World Expo in 2005. The Homestar Planetarium can be carried in a bag and is intended for home use, however by projecting 10,000 stars on the ceiling makes it semi-professional.
 Planetarium technology
Planetarium domes range in size from 3 to 30 m in diameter, accommodating from 1 to 500 people. They can be permanent or portable, depending on the application.
- Portable inflatable domes can be inflated in minutes. Such domes are often used for touring planetariums visiting, for example, schools and community centres.
- Temporary structures using Glass-reinforced plastic (GRP) segments bolted together and mounted on a frame are possible. As they may take some hours to construct, they are more suitable for applications such as exhibition stands where a dome will stay up for a period of at least several days.
- Negative pressure inflatated domes are suitable in some semi-permanent situations. They use a fan to extract air from behind the dome surface, allowing atmospheric pressure to push it into the correct shape.
- Smaller permanent domes are frequently constructed from glass reinforced plastic. This is inexpensive but, as the projection surface reflects sound as well as light, the acoustics inside this type of dome can detract from its utility. Such a solid dome also presents issues connected with heating and ventilation in a large-audience planetarium, as air cannot pass through it.
- Older planetarium domes were built using traditional construction materials and surfaced with plaster. This method is relatively expensive and suffers the same acoustic and ventilation issues as GRP.
- Most modern domes are built from thin aluminium sections with ribs providing a supporting structure behind. The use of aluminium makes it easy to perforate the dome with thousands of tiny holes. This reduce the reflectivity of sound back to the audience (providing better acoustic characteristics), lets a sound system project through the dome from behind (offering sound that seems to come from appropriate directions related to a show), and allows air circulation through the projection surface for climate control.
The realism of the viewing experience in a planetarium depends significantly on the dynamic range of the image, i.e., the contrast between dark and light. This can be a challenge in any domed projection environment, because a bright image projected on one side of the dome will tend to reflect light across to the opposite side, "lifting" the black level there and so making the whole image look less realistic. While traditional planetarium shows consisted mainly of small points of light (i.e., stars) on a black background this was not a significant issue, but it became an issue as digital projection systems started to fill large portions of the dome with bright objects (e.g., large images of the sun in context). For this reason, modern planetarium domes are often not painted white but rather a mid grey colour, reducing reflection to perhaps 35-50%. This increases the perceived level of contrast.
A major challenge in dome construction is to make seams as invisible as possible. Painting a dome after installation is a major task and, if done properly, the seams can be made almost to disappear.
Traditionally, planetarium 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. Tilted domes generally have seating arranged 'stadium-style' in straight, tiered rows; horizontal domes usually have seats in circular rows, arranged in concentric (facing center) or epicentric (facing front) arrays.
Often around the edge of the dome (the 'cove') are:-
- Silhouette models of geography or buildings like those in the area round the planetarium building.
- Lighting to simulate the effect of twilight or urban light pollution.
- In one planetarium the horizon decor included a small model of a UFO flying.
Traditionally, planetariums needed many incandescent lamps around the cove of the dome to help audience entry and exit, to simulate sunrise and sunset, and to provide working light for dome cleaning. More recently, solid-state LED lighting has become available that significantly decreases power consumption and reduces the maintenance requirement as lamps no longer have to be changed on a regular basis.
 Traditional electromechanical/optical projectors
Traditional planetarium projection apparatus uses a hollow ball with a light inside, and a pinhole for each star, hence the name "star ball". With some of the brightest stars (e.g. Sirius, Canopus, Vega), the hole must be so big to let enough light through that there must be a small lens in the hole to focus the light to a sharp point on the dome.
The star ball is usually mounted so it can rotate as a whole to simulate the Earth's daily rotation, and to change the simulated latitude on Earth. There is also usually a means of rotating to produce the effect of precession of the equinoxes. Often, one such ball is attached at its south ecliptic pole. In that case, the view cannot go so far south that any of the resulting blank area at the south is projected on the dome. Some star projectors have two balls at opposite ends of the projector like a dumbbell. In that case all stars can be shown and the view can go to either pole or anywhere between. But care must be taken that the projection fields of the two balls match where they meet or overlap.
Smaller planetarium projectors include a set of fixed stars, Sun, Moon, and planets, and various nebulae. Larger projectors 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. Others add coordinate lines and constellations, photographic slides, laser displays, and other images.
Each planet is projected by a sharply focused spotlight that makes a spot of light on the dome. Planet projectors must have gearing to move their positioning and thereby simulate the planets' movements. These can be of these types:-
- Copernican. The axis represents the Sun. The rotating piece that represents each planet carries a light that must be arranged and guided to swivel so it always faces towards the rotating piece that represents the Earth. This presents mechanical problems including:-
- The planet lights must be powered by wires, which have to bend about as the planets rotate, and repeatedly bending copper wire tends to cause metal fatigue.
- When a planet is at opposition to the Earth, its light is liable to be blocked by the mechanism's central axle.
- (If the planet mechanism is set 180° rotated from reality, the lights are carried by the Earth and shines towards each planet, and the blocking risk happens at conjunction with Earth.)
- Ptolemaic. Here the central axis represents the Earth. Each planet light is on a mount which rotates only about the central axis, and is aimed by a guide which is steered by a deferent and an epicycle (or whatever the planetarium maker calls them). Here Ptolemy's number values must be revised to remove the daily rotation, which in a planetarium is catered for otherwise.
- Computer-controlled. Here all the planet lights are on mounts which rotate only about the central axis, and are aimed by a computer.
Despite offering a good viewer experience, traditional star ball projectors suffer several inherent limitations. From a practical point of view, the low light levels require several minutes for the audience to "dark adapt" its eyesight. "Star ball" projection is limited in education terms by its inability to move beyond an earth-bound view of the night sky. Finally, a challenge for most traditional projectors is that the various overlaid projection systems are incapable of proper occultation. This means that a planet image projected on top of a star field (for example) will still show the stars shining through the planet image, degrading the quality of the viewing experience. For related reasons, some planetariums show stars below the horizon projecting on the walls below the dome or on the floor, or (with a bright star or a planet) shining in the eyes of someone in the audience.
 Digital projectors
An increasing number of planetariums 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 planetariums 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 "fish eye 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, making their images subjectively indistinguishable from the very best "star balls" to most eyes.
LCD projectors have fundamental limits on their ability to project true black as well as light, which has tended to limit their use in planetariums. 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.
 Planetarium show content
Worldwide, most planetariums 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 (despite the increased expense) because a live expert presenter can answer on the spot questions raised by the audience.
Since the early 1990s, fully featured 3-D digital planetariums 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.
 Images of planetariums
James S. McDonnell Planetarium In St. Louis, Missouri
Cruise liner Queen Mary 2 Planetarium
 Images of planetarium projectors
 Notable planetariums
- Britain and Northern Ireland:
- Armagh Planetarium, , Armagh, Northern Ireland
- London Planetarium, Marylebone Road, London (part of Madame Tussaud's), closed in 2006.
- Scottish Power Planetarium, Glasgow Science Centre, Glasgow, United Kingdom
- At Jodrell Bank, closed in 2003.
- At the National Space Centre in Leicester, but only used as a cinema.
- European Union:
- Silesian Planetarium, Katowice, Poland
- Artis Planetarium, Amsterdam
- Athens Planetarium, Athens, Greece
- Eise Eisinga Planetarium, Franeker, Friesland, Netherlands, 1774
- Planetarium Galilee, Montpellier, France
- Lisbon Gulbenkian Planetarium, Praça do Império, Lisbon, Portugal
- Särkänniemi Planetarium, Tampere, Finland
- Hamburg Planetarium, Hamburg, Germany
- Osservatorio Astronomico Di Torino, Turin, Italy
- Cite de l'espace,Toulouse, France
- Nuremberg Planetarium, Nuremberg, Germany
- Abrams Planetarium, Michigan State University, East Lansing, Michigan, Michigan
- Adler Planetarium, Chicago, Illinois
- Alden Planetarium, EcoTarium, Worcester, Massachusetts
- Albert Einstein Planetarium, National Air and Space Museum, Smithsonian Institution, Washington, DC
- Cernan Earth and Space Center, Triton College, River Grove, Illinois
- Clark Planetarium, Salt Lake City, Utah
- CyberSphere Digital Theater, Dickson, TN, The Renaissance Center
- Dassault Systemes Planetarium at the New Detroit Science Center
- Davis Planetarium at the Maryland Science Center
- Delta College Planetarium & Learning Center Bay City, MI
- Dreyfuss Planetarium at The Newark Museum, Newark, NJ
- EpiSphere at the Aerospace Education Center, Little Rock, Arkansas - first single-projector digital planetarium; one of only three so far
- Fels Planetarium at the Franklin Institute (Philadelphia, Pennsylvania)
- Gheen's Science Hall & Rauch Planetarium, University of Louisville, Louisville, Kentucky, 
- George Alden Planetarium at the Ecotarium (Worcester, Massachusetts)
- Griffith Observatory, Los Angeles, California
- Charles Hayden Planetarium at the Museum of Science, (Boston, Massachusetts)
- Framingham State College Planetarium Framingham, Massachusetts
- Irene W. Pennington Planetarium, Baton Rouge, Louisiana
- Lodestar Astronomy Center, Albuquerque, New Mexico
- Longway Planetarium, Flint, Michigan
- Miami Museum of Science & Planetarium, Miami, Florida, opened in 1966.
- Minneapolis Planetarium, Minneapolis Public Library, Minneapolis, Minnesota. Until the MPL's central branch was closed and demolished in 2002, the Minneapolis Planetarium had the oldest extant projector (installed in 1954); the fate of that projector is unknown. A new public library opened in its place in 2006, and a new planetarium with modern digital projection capabilities is planned to be added to the building in 2009.
- Morehead Planetarium and Science Center at the University of North Carolina at Chapel Hill is the first planetarium that was built on a U.S. college campus.
- Rosicrucian Egyptian Museum, in San Jose, California, which has a purpose-built planetarium rendered in an Ancient Egyptian architectural style
- Shafan Planetarium] at the Cleveland Museum of Natural History, Cleveland, Ohio
To give some idea of the number of planetariums and the difficulty in trying to list all, over fifty have been documented as having been sold to various locations in the five boroughs of New York City, ranging from one in Manhattan with a 76-foot dome that is used as a light effect in a disco to a two elementary schools in the Bronx with 12-foot domes. There are also many portable planetariums, including two on Staten Island that are privately owned, plus one in an Intermediate School. A little known model, the Aquarian, was made in the 1970s. Only about twenty are believed to have been sold, but only one of these has been tracked down, stored under the auditorium floor of an elementary school in Queens.
- The rest of the world:-
- Nehru Planetariums, one each at Mumbai, New Delhi, and Bangalore, Leo Planetaria at New Delhi, India
- Planetario Alfa Monterrey, México
- Ehime Prefectural Science Museum, Ehime, Japan has one of largest domes in the world (30m in diameter)
- Kobe Science Museum, Kobe, Japan
- Taipei Astronomical Museum, Taipei, Taiwan
- Hong Kong Space Museum, Kowloon, Hong Kong, China
- Kuching Planetarium, Sarawak, Malaysia
- Planetarium Sultan Iskandar Malaysia
- PIA planetarium, Karachi and Lahore
- Johannesburg Planetarium
- Planetario Galileo Galilei, Buenos Aires, Argentina
- [Planetario "Simón Bolívar], Maracaibo, Venezuela
- [Planetario Humtbold], Caracas, Venezuela
- Planetario Ciudad de San José San José, Costa Rica
 Planetarium computer software
Since the invention of the personal computer, the planetarium concept has been extended to include software that displays a realistic sky image. Not all astronomy software can be considered to be planetarium software. While there is no generally-accepted definition of the word planetarium in this context, planetarium software generally contains the following features at a minimum:
- A realistic sky image as seen from Earth
- The ability to display sky motions in real-time
- The ability to animate time changes backwards and forwards
The following is a list of currently available planetarium software:
- Aladin Sky Atlas  (Java)
- Asynx Planetarium (Windows)
- Cartes du Ciel   (Linux, Windows)
- Celestia  (Linux, Windows, Mac OS X; successor of 3DPlanetarium, OpenUniverse)
- Digistar 3 (proprietary hardware, Windows XP Pro, Evans and Sutherland : Digital Theater Division)
- DigitalSky (proprietary hardware, Windows XP Pro, Sky-Skan)
- Distant Suns  Windows
- Earth Centred Universe  Windows
- InSpace System (proprietary hardware, RSA Cosmos)
- KStars  (Linux)
- Planetarium  Palm OS
- RedShift  (Windows)
- Skyglobe  (MS-DOS, Windows)
- StarStrider (Windows)
- Starry Night (Windows, Mac OS X)
- Stellarium (Linux, Windows, Mac OS X)
- TheSky  (Windows)
- Winstars  (Windows)
- XEphem (Linux, FreeBSD, Mac OS X, Solaris, AIX, HP-UX , Windows with Cygwin)
 See also
- Antikythera mechanism
- Armillary sphere
- Astronomical clock
- Fulldome video
- Prague Orloj
- Star atlas
- King, Henry C. "Geared to the Stars; the evolution of planetariums, orreries, and astronomical clocks" University of Toronto Press, 1978
 External links
- List of Planetarium Software
- International Planetarium Society
- Loch Ness Productions list of planetariums worldwide
- Sky-Skan, Inc. - Digital planetarium system manufacturer
- Map: Planetaria and Observatories
- RSA Cosmos manufacturer of Digital and Optical Planetarium
- Article about Sega Homestar Planetarium