Diagram MAPD0297

Partes Angustantur: The diagram represents the cosmological and geometrical fact that an object or an arc of a given metrical length will fill a smaller and smaller angle as it is removed from the center. In the text created for this diagram the emphasis is upon the narrowing of angular arcs, and thus of planetary orbits, to shorter and shorter lengths as one approaches the central earth. This diagram can also function as an optical diagram to show that two arcs of equal length traced by two radii of different lengths will appear under different visual angles. I is worth noting that this reasoning also appears near the end of the Plinian excerpt (see above) "De Absidibus earum": nearer planets move faster, and motion at apogee is slowest. Two concentric circles are connected by a number of radial line segments, usually sixteen or more (not twelve). The only elaboration in the diagram is the label written circularly within the outer circle. In all but one case this label is "partes angustantur iuxta terram".