Why is M87 jet one sided in appearance?

The jet in M87 is one sided and knotty. The question of whether the one sidedness is due to relativistic beaming or to intrinsic one sidedness has not yet been resolved. Recently, the knots have been observed in detail at the Very Large Array (VLA)¹; knot A is observed to consist of a thin disk, apparently a shock that is perpendicular to the jet axis and viewed edge-on, where the radio brightness profile has a very sharp gradient, and also of a trail downstream of the disk, that is pointing away from the nucleus. We discuss here the geometric implications of these observations, in particular the sharp edge facing the nucleus. We discuss models that represent the extremes in the apparent motion of the shock, and argue that proper motion measurements of the knot's inner edge would discriminate between them.