It is not easy to disassociate oneself from the observational perspective on the Earth, and relate to the apparent movements of the stars and the Earth as an independent observer. Models overcome this difficulty by mechanically realizing certain assumptions about the actual movements of the stars. An observer of the model is then, to a certain extent, situated outside of the moving celestial bodies at a location independent of these movements.

Models of this kind enjoy great popularity since the disputes about Copernicanism in the 17th century. In the following centuries, precision engineering workshops manufacture numerous telluria, planetaria and cosmographic clocks used for representative purposes or for teaching astronomy.

C. F. Delamarche, Les usages de la sphère, et des globes céleste et terrestre