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Planetariums
 

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. 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, 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 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.

History

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. These devices would today usually be referred to as orreries (named for the Earl of Orrery, a location in Ireland--an 18th century Earl 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 through Saturn) going around it in something close to their correct relative periods.

The first electromechanical/optical planetarium projectors were designed and built by Carl Zeiss in 1924 Germany, on a suggestion by the German astronomer Wolf, and have grown more complex. A great boost to their installation 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 planetaria in US high schools.

During the 1970s, the OmniMax movie system (now known as IMAX Dome) was conceived to operate on planetarium screens. More recently, some planetaria — have re-branded themselves as dome theaters — with broader offerings including wide-screen or "wraparound" 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, RSA Cosmos's InSpace System or Sky-Skan's DigitalSky, offer a fully digital projection system. 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).

Outside the permanent installations of museums and science centres, portable planetariums are often set up for education programs in locations such as schools.

Prior to World War 2 nearly all planetariums were built by Zeiss, the only notable exceptions being two built by two brothers named Korkosz based in Boston.

After the war not only was Germany split, but so was the Zeiss firm. Part remained in the traditional headquarters of Jena, now in East Germany, while 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, while 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 this planet 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.

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 capitalize 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.

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 placed 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.

When Germany reunified, the two Zeiss firms did likewise, and expanded their offerings to cover many different size domes. Meanwhile, Evans and Sutherland in Utah became the first firm to offer a planetarium whose projections were created by a computer. StarLab in Massachusetts offered the first easily portable planetarium in 1977 (Viewlex had a portable version of the Apollo earlier) which projected stars, constellation figures from many mythologies, celestial coordinate systems, and much else, from removable cylinders.

Planetariums have become while 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 23 meter dome seating 720 people of the Hayden Planetarium, to 3.8 meter inflatable, portable domes where children sit on the floor.

Planetarium technology

Domes

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.

  • Amongst portable domes, inflated structures can be pumped up in a matter of minutes. Such domes are often used for touring planetaria visiting, for example, schools and community centres.
  • Temporary structures using Glass-reinforced plastic (GRP) segments bolted together and mounted on a frame are also 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 with as fresh air cannot be introduced 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 a better acoustic), allows a sound system to project through the dome from behind (offering sound that appears to come from appropriate directions related to a show) and it also allows heating and ventilation to take place through the projection surface.

The realism of the viewing experience in a planetarium depends significantly on the dynamic range of the image, ie the perceived 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 planetaria shows consisted mainly of small points of light (ie 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 (eg 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, 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.

Digital planetaria frequently now include controls such as buttons or joysticks in the arm-rests of seats to allow audience feedback that influences the show in 'real time'.

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, planetaria required many incandescent lamps around the cove of the dome to facilitate 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 star ball 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.

Each planet is projected by means of 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 light for each planet 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.
When a planet is at opposition to the Sun, its light is liable to be blocked by the mechanism's central axle.
  • 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. If the planetarium includes a Uranus planet spotlight (as occasionally happens), that needs working out a Ptolemaic system mechanism for Uranus.
  • 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 planetaria 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 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 are mixing both traditional electromechanical/optical 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 CRT, 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 planetaria. "Dark chip" DLP projectors overcome this limitation and can offer relatively inexpensive solution with bright images that require less time for an audience to "dark adapt" than in a traditional planetarium. For the time being, CRT projectors offer slightly better contrast than DLP and also project a wider range of colours, though the image is not as bright as that from DLP systems, CRT tubes require eventual replacement and the equipment is much bulkier than a DLP projector of similar specification. As the technology matures and reduces in price, laser projection looks set to become the ultimate technology for dome projection as it offers bright images, large dynamic range and a very wide color space.

    Planetarium show content

    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 linked to the night sky have been popular. Pre-recorded and live presentation formats are possible with the latter preferred by many venues (despite the increased expense) because a live expert presenter can answer questions raised by the audience on the spot.

    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 just the earth-bound view with which we are most familiar. This new virtual reality-capability to travel through the universe provides important educational benefits because 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 and instead to 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, revealing 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 especially visual or spatially-aware people, this experience can be more educationally beneficial than other demonstrations.

    Notable planetariums

    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 couple of 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 a third 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.

    Planetarium computer software

    See also

    External links

    Theatre | Planetaria | Observation

    Planetarium | Planetarium | Planetario | Planétarium | Planetario | פלנטריום | Planetarium | プラネタリウム | Planetário | Planetaario | Planetarium

     

    This article is licensed under the GNU Free Documentation License. It uses material from the "Planetarium".

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