Message #1674

[Majir]

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 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 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. (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 "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, and are far superior to any digital star projector.