In the previous chapter we have provided the means of accurately calculating the positions of the major planets. Anyone who frequently observes the planets, the Sun, or the Moon, is also interested in data that describe the aspect of the various bodies. The visual magnitude and the apparent angular diameter are data of fundamental importance. Anyone who wants to observe and plot surface markings or atmospheric structure also needs to know the orientation of the planet’s axis of rotation (and thus of the planetographic coordinate system), relative to the line of sight, together with the conditions of illumination. We intend to show how these data may be calculated. In doing so, we shall try to achieve an accuracy that should suffice, in most instances, for practical observations.