Diagram MAPD0028

Generic Epicycle: The manuscript diagrams for the generic epicycle follow generally the description of Calcidius's text. The most common divergence from the text's description is a failure of the radii tangent to the epicycle to be, in fact, tangent. Calcidius's account here might not be clear to a reader not already familiar with such a diagram. The purpose of the diagram is to identify the forward and retrograde arcs of the epicycle as well as the stationary points of the planet on the epicycle. The zodiacal circle and the earth at its center are specified first. The epicyclic circle EZH is then presented, but the text requires a preexisting diagram and simply discusses the motion of points on the diagram. The three points labeling the epicycle only become clear as the description proceeds, the points Z and H being the points of tangency with the two radii drawn to the zodiacal circle and the point E being between the zodiacal point A and the center M of the epicycle along a radius from the earth through M to A. Once this figure is recognized, the reader can follow Calcidius's account of epicyclic motion, which he applies here to any planet and gives no specifics of planetary motion. The epicycle here is fixed and does not move around the zodiac. The epicycle only rotates, with its motion in the same sense as the celestial sphere, from east to west, showing the motion of a planet on its periphery. Calcidius point out the two locations, or regions, of planetary station in two very small arcs including the tangent points H and Z. He then labels the passage from Z through E to H as the proper motion of the planet on the periphery of the epicycle and the passage from H to Z as the retrograde motion of the planet. He continually reminds the reader that the planetary motions are seen against the background of the zodiac rather than directly on the epicyclic circle.