A planetarium (plural form: planetariums or planetaria) is a theater 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, such as precision-engineered 'star balls' that combine optical and electro-mechanical technology, slide projector, video and fulldome projector systems, and lasers. Whatever the technologies used, they are combined to provide a display of relative motions of objects in 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 have become nearly ubiquitous, and some are privately owned. According to a rough estimate, the United States has 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 in which children sit on the floor. Such portable planetariums serve educational programs outside of the permanent installations of museums and science centers.
Archimedes is attributed with possessing a primitive planetarium device that could predict the movements of the Sun, Moon, and 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 eighteenth-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 eighteenth-century orreries limited their impact, and toward 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 built 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. He 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. It was displayed at the Deutsches Museum in 1924, construction work having been interrupted by the war. The planets traveled 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. The design was 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 that could 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.
Before World War II, nearly all planetariums were built by Zeiss. The notable exceptions included one built by two brothers named Korkosz in Springfield, Massachusetts, and another for the Rosicrucian AMORC order in San Jose, California.
When Germany was divided into East and West Germany after the war, the Zeiss firm was also split. Part remained in its traditional headquarters at Jena, 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 operated from 1952 to 2003. The Korkosz brothers built a large projector for the Boston Museum of Science, which was unique in being the first (and for a long time only) planetarium to project the planet Uranus. Most planetariums ignore Uranus as being at best marginally visible to the naked eye.
Planetarium popularity got a worldwide boost by the Space Race of the 1950s and 60s. In particular, fears that the United States 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.
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 the 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 that 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 programs were prepared for various grade levels and the public, but operators could also 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. It projected stars, constellation figures from many mythologies, celestial coordinate systems, and much else from removable cylinders. Viewlex and others followed with their own portable versions.
After German reunification in 1989, the two Zeiss firms did likewise and expanded their offerings to cover different-sized domes.
In 1983, Evans & Sutherland installed the first planetarium projector displaying computer graphics—the Digistar I projector used a vector graphics system to display starfields as well as line art.
The newest generation of planetariums, such as Evans & Sutherland's Digistar 3, RSA Cosmos's InSpace System, Konica Minolta's MEDIAGLOBE, or Sky-Skan's DigitalSky, offer a fully digital projection system, using fulldome video technology. This gives operators great flexibility in showing not only the modern night sky as visible from Earth, but also any other images they choose, 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 has an international reputation 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, it is classified as semi-professional.
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.
The realism of the viewing experience in a planetarium depends significantly on the dynamic range of the image, that is, 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. Since 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 gray color, 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 favored '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.
Planetariums occasionally include controls such as buttons or joysticks in the arm-rests of seats to allow audience feedback that influences the show in real time.
The edge of the dome (the 'cove') may have lighting to simulate the effect of twilight or urban light pollution, or silhouette models of structures in the area round the planetarium building.
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 the lamps no longer have to be changed on a regular basis.
Traditional planetarium projection apparatus uses a hollow ball with a light inside, and a pinhole for each star, hence the name "star ball." To show some of the brightest stars (such as 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 such that 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 the following types:
Despite offering a good viewer experience, traditional star ball projectors have several inherent limitations. From a practical point of view, the low light levels require several minutes for members of the audience to "dark adapt" their 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.
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 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 for such systems compared with traditional "star balls," noting that they employ few moving parts and do not generally require synchronization 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, liquid crystal display (LCD), digital light processing (DLP), or laser projectors. Sometimes, a single projector mounted near the center of the dome is employed with a "fish eye lens" to spread the light over the whole dome surface. 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. Although 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.
However, these digital star projectors do not show "pinpoint" stars as one would observe in the real sky. Also, the colors of the stars are not always correct. Although digital projectors are good for "traveling" through space, their ability to show a realistic star field is years away. Also, some say that maintenance costs of the digital and video units are significantly higher than those of their optical-mechanical counterparts.
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 (liquid crystal on silicon) 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 a relatively inexpensive solution with bright images, but the black level requires physical baffling of the projectors. As the technology matures and prices drop, laser projection seems promising for dome projection because it offers bright images, large dynamic range and a very wide color space.
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 that 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 formats are preferred by many venues (despite the increased expense) because members of the audience can get immediate answers from an expert presenter.
Since the early 1990s, fully featured 3-D digital planetariums have added an extra degree of freedom to a presenter because they allow simulation of the view from any point in space, not just the earth-bound view that we are most familiar with. This new virtual reality capability to travel through the universe provides important educational benefits: It vividly conveys that space has depth, helping audiences leave behind the ancient misconception that the stars are stuck on the inside of a giant celestial sphere, and to understand the true layout of the Solar System and beyond.
For example, a planetarium can now 'fly' the audience toward one of the familiar constellations such as Orion, revealing that the stars that appear to make up a coordinated 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.
James S. McDonnell Planetarium in St. Louis, Missouri
Kuching Planetarium in Sarawak, Malaysia
Alfa Planetarium, Monterrey N.L.Mexico
Griffith Obsevatory's central dome contains a planetarium, in Los Angeles, California
All links retrieved May 6, 2015.
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