The PRC is divided into four categories, based on the certainty of the identification. This identification is difficult because a polar-ring galaxy is only obvious if it is seen in a certain orientation on the sky. A graphic illustration of this problem is shown in Figure 1, in which a flat disk and an orthogonal ring, whose inner radius is the same size as the outer radius of the disk, are shown from a variety of viewing orientations. The angle is the amount the ring has been rotated, while the angle is the amount the disk was subsequently rotated. Finally, the model is silhouetted to depict a two-dimensional observation on the sky. Only when both ring and disk are seen relatively edge-on is the identification obvious. If the projection angle on the sky is not optimal, a polar-ring galaxy can masquerade as an Sa galaxy (e.g., upper left of Fig. 1) or a barred "theta" galaxy (e.g., lower left of Fig. 1). These cases of mistaken identity could generally be distinguished by detailed kinematic and photometric observations. This figure also indicates that for every polar-ring galaxy we identify, there are one or two others that have been missed because of their orientation on the sky. The criteria used to categorize the galaxies in the PRC are described below.
Figure 1. The appearance of a polar-ring galaxy as seen from a variety of viewing orientations. Note that less than half of the viewing orientations allow easy identification of the system as a polar-ring galaxy.