Copernicus calculated with respect to the distances and periods of Mercury as follows. Consider the diagram in Fig. 69a. The inside circle represents Mercury’s orbit; the outer circle, the earth’s. Clearly, from the earth Mercury should always be seen close to the sun ‘oscillating’ to either side of it. E1 and M1 denote the earth and Mercury when the latter is farthest to the west. The angular distance between the two bodies at this time averages out as about 23°.