Editing Planetarium (section)

===Traditional electromechanical/optical projectors===
{{Main|Planetarium projector}}
{{Multiple image|perrow=2|total_width=300
|image1=Bundesarchiv B 145 Bild-P018935, Berlin, Planetarium.jpg|caption1=A [[Zeiss projector]] in a Berlin planetarium during a show in 1939.
|image2=ZeissPlanetariumProjector MontrealPlanetarium.jpg|caption2=Zeiss projector at [[Dow Planetarium]] in Montreal
|image3=Universarium in Planetarium Hamburg.jpg|caption3=A modern, egg-shaped Zeiss projector (UNIVERSARIUM Mark IX) at the Hamburg planetarium
|image4=Киевский планетарий. Аппарат "Большой цейс - 4".JPG|caption4=Zeiss projector at [[Kyiv Planetarium]]
}}
Traditional [[planetarium projector|planetarium projection apparatus]] use 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. In later and modern planetarium star balls, the individual bright stars often have individual projectors, shaped like small hand-held torches, with focusing lenses for individual bright stars. Contact breakers prevent the projectors from projecting below the "horizon".{{Citation needed|date=October 2008}}

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 [[nebula]]e. Larger projectors also include [[comet]]s 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 [[constellation]]s, photographic slides, [[laser]] displays, and other images.

Each planet is projected by a sharply focused [[Stage lighting instrument#Spotlights|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:-
*[[Heliocentrism|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 wire breakage through [[metal fatigue]].
*:When a planet is at [[Opposition (astronomy)|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 shine towards each planet, and the blocking risk happens at [[conjunction (astronomy)|conjunction]] with Earth.)
*[[Geocentric model|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. (In one planetarium, this needed Ptolemaic-type orbital constants for [[Uranus]], which was unknown to Ptolemy.)
*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 [[Adaptation (eye)|"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, in most traditional projectors 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.