1682: An ingenious mechanism for a first moving model of planets was conceived by the Dutch physicist Christiaan Huygens (1629-1695). He took the ratio of the orbital periods of two planets and applied the method of continued fractions. He got an approximation for the construction of the cog wheels. For example, the ratio of the periods of Saturn and Earth is given by 77.708.431 : 2.640.858. The expansion of the continued fraction yields an approximation of 206:7. Thus, if the cogwheels of Saturn and the Earth had 206 and 7 cogs, respectively, then their modelled times would stay as commensurable as possible. A deviation only occurred after 1346 revolutions of the Earth. This was the first properly functioning miniature planetarium.

In the Huygens' version the planets run on circular rails around the sun. Their orbits are slightly displaced against the centre, in order to suggest the ellipticity. They are driven by one single diagonal axis. The apparatus is stored in a octogonal box 70 cm wide. The Huygens' planetarium was fabricated by the clockmaker Johannes van Ceulen (< 1650 -1715). The image displays the backside of the clockwork.