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Definition of Planetarium Categories PDF Print

Computer sketch of a dome designThe realisation of the modern planetarium comes in many shapes and sizes, ranging from a small umbrella-like facility of hardly 3 meters to huge domes providing space for several hundred visitors. Such a planetarium may shelter an opto-mechanical star projector (the classical style) as well as a number of video projectors for a digital presentation (virtual style). Difficulties arise when evaluating a planetarium's prominence, especially, when seen through the eyes of the public, who are accustomed to classifying them to "large", "middle", and "small" buildings. A closer look at the parameters, that determine a planetarium, reveals that only two are stable against arbitrariness: the diameter of the dome and the type of the projector. The number of seats, often mentioned in the planetarium's description, turns out to be of little significance.

Histogram of dome sizesA statistics of French, German and Italian planetariums in 2007 defined clear boundaries for their categories. The combined histogram of the national listings suggests empirically that most of the planetaria fill a regime below 10 meters (orange colour). They usually harbour an easy-type star projector. Another distinct peak can be perceived at the regime above 18 meters (blue). These planetaria are accounted as "large" ones. The gap inbetween is filled by the medium-sized domes (green). A similar histogram based on the number of seats is tampered towards the lower values, for the nominal capacity might have been enlarged by addiditional chairs, or they provide benches without marked-off spaces. The survey was cross-checked with the projector performance, in particular its power output. The correlation between projector model and the dome size yields that the categories are reflected surprisingly well by the projector. Of course, it does not reveal the exact diameter, but inserts almost each planetarium into the same category as obtained by the dome size.

The full article was published in: Planetarian 36, 2007-4, p12-13.


Category diameter seats # in D
large planetariums > 18 m > 200 9
medium-sized planetariums 10 - 17 m 75 - 199 11
small planetariums   5 -  9 m 30 - 74 55
school planetarium  (< 5 m)  < 30 15