The opto-mechanical projectors achieved a great development in the course of the decades. The very first artificial skies were reproduced by the illumination of small pin holes of different size in the metallic plates. The quality of the stars improved with the deployment of photographic plates, and then with fibre optics. The best projectors copy the night sky very well - close to perfection. However, it is not as straight as the real sky. A trained observer may find small demerits like inconsistency in brightness, dislocation of planets, or unprecise moon phase.

The reason for that is that the orbits of the planets are ellipses in nature and subject to a variable velocity (3rd Keplerian Law). Moreover, they have inclinations against the ecliptic and varying distances to the Earth. However, the star projector embodies cogwheels and motors with an approximate mechanics in order to place the dot on the dome. Although those natural circumstances will be considered as much as possible by the gear, it still does not reflect absolute correctness. One finds that planets would not be exactly in accord to the "background" stars, or the angle of view with other planets might be wrong. Planetary encounters reveal such irregularities, in particular. The same holds for eclipses. For example, lunar eclipses are completely neglected. Such deviations are tiny in the modern, computer-controlled projectors, but the planetarium's sky is not to be mirrored one-to-one in any case.