Planetaria are sometimes met in a complex with observatories or museums. The observatory has also a dome, so, what does actually distinguish it from a planetarium?

The dome of the observatory serves just as shelter for the optical instruments against the exposition to the elements. When the cupola is opened, one gazes at the real sky under free-field conditions. A telescope will be positioned on a certain object, and only one person is able to look through. The field of view is restricted to the particular section of the sky around the object. Guidences at the observatory are due to the weather conditions.

In the planetarium, the sky is generated artificially. A special projector maps the stars on the inside of the dome which cannot be opened. The motions are replicated, i.e. the monthly course of the heavenly bodies is reduced to a few seconds. The projector is a kind of "time machine" to calculate the configurations of the sun, moon and the planets in the future or past. Also, the position of the observer can virtually be relocated to any place on Earth. Some newer, computer controlled devices also enable the sight from other planets. The visitors experience a star show which may be arranged live by a presenter or played by a tape.