A planetarium projector is a device capable of projecting, onto the inside reflecting surface of a hemispherical dome, a realistic view of the night sky as seen from any place on the surface of the Earth. It is also able to demonstrate all the major movements associated with the sky. These include the daily (diurnal) motion of the sky, the monthly motion of the Moon (including its phases), the annual motion of the Sun and planets against the background of the stars, and the precessional motion of the Earth’s axis. There are separate projectors for the stellar background, the Sun, the Moon and the planets.
One of the simplest ways of obtaining the stellar background is by means of a sphere with many small holes drilled into its surface. The sizes of the holes are related to the brightness of the stars, and they are positioned so as to represent the stars in their correct relative positions. At the center of this sphere is a point source of light, giving sharp images without lenses, mounted in gimbals. A hemispherical cup cuts out light for stars which lie below the horizon, no matter which way the projector is pointing.
Light from the point source passes through the holes in the sphere, and causes spots of light to fall on the inside of the dome. The holes for the bigger stars must be large, so these stars would appear as discs instead of points of light. To overcome this difficulty the larger stars have small lenses whichconcentrate the light to form points. This type of system is used in the Spitz projector, which uses a mercury – xenon arc light source.
The Zeiss projector uses a different system. This projector has a dumbbell shape. Each end of the dumbbell has 16 small projectors, and each has its own optical system. These projectors are so grouped around a central 1000-watt bulb that together they can project a complete picture of the stellar background over the whole surface of the dome. Inside each projector there is a photo-engraved plate of part of the sky. Mechanical blinkers swing over the projection lenses like artificial eyelids, thus preventing star images which are below the horizon from flashing across the eyes of the spectators.
Most projectors are able to move about three independent axes, to allow for the fact that the observer can be at any latitude, at any time of day, and at any era in time. The Earth’s axis moves slowly like that of a spinning top or gyroscope – a movement called precession, which has to be allowed for.
Two different methods can be used to simulate the motions of the Sun, the Moon and the planets, which move along different paths in the sky at varying rates. In one system, the projectors themselves do the movement, while in the other the light from the projectors reaches the screen via a system of movable mirrors. The movements of the Sun and Moon present little difficulty. The inclination of their orbits to the celestial equator (the projection of the terrestrial equator onto the sky) is achieved by means of cylindrical wedges. The Moon projector has an automatic shutter which simulates the phases of the Moon.
The movements of the planets are simulated by means of analogs. These are mechanical versions of the solar system which contain the Sun, the Earth and one of the planets. The Earth and the planet move around the Sun at the correct relative speeds, and at the appropriate distances. The planet and the Earth are connected to the same rod by means of sliding contacts. This rod represents the line of sight from the Earth to the planet. In the first system thisrod carries the projector, and in the second system the rod controls the movement of a mirror.
Auxiliary projectors can project onto the dome such things as coordinate systems, the meridian, the ecliptic (or the apparent pathway of the Sun against the background stars) and a view of the continents as seen from the center of the transparent Earth, for navigation instruction. Some planetariums also have projection orreries which project onto the dome a view of the solar system, including the Earth, as seen from deep in space.