The Moments of the Brightness Distribution of a Radio Source

It is often desirable to describe the overall brightness distribution of a radio source by parameters which do not involve assumptions about the form of the of parameters, and their use is discussed with reference to (i) a two-dimensional map, and (ii) the distribution of polarization. The description of a source by its moments, while supplementing rather than replacing the description by means of a map, is particularly relevant in cases where the source is close to the limit of resolution available. It is shown in an Appendix that if the brightness distribution is obtained from interferometric data, the zero, second and fourth moments are not affected by phase irregularities, such as may be caused by atmospheric seeing. The brightness distribution and polarization of the source 4C 25·33 is discussed by way of an example.