Number distribution of the measured tangential velocities (in km s^-1, assuming d = 415 pc). The average velocities over a 10-year timespan from 2003 to 2013 (lower panel) are compared to the more “instantaneous” velocities measured on the shortest timescales of about 2 years (upper panel), the 2011−2013 period (bold red) and the 2009−2011 period (blue). Only knots with proper motion velocities above 2σ (vertical dashed lines) are included.