Astronomical compendium This astronomical compendium, in the shape of a Missal, carries the coat of arms of the Company of Jesus (IHS). The outer face of the lid bears a lunar dial showing the phases of the moon; the inner face is engraved with the hour lines. Inside, there is a tilting gnomon mounted on a compass (now missing), that ensured the instrument's correct orientation and allowed its use as a dial. The back of the book displays the planetary hours.
This early Persian astrolabe with a geared calendar movement is the oldest geared machine in existence in a complete state. It illustrates an important stage in the development of the various complex astronomical machines from which the mechanical clock derives.