Comparison of water maser observations at different epochs. The left and right panels present an expanded view of the water maser clusters 1 and 4, respectively. The colored triangles and squares have the same meaning as in Fig. 2. We use the same symbol size for all the masers to better emphasize their relative spatial distribution. The black-edged pentagons report the position of the water masers observed with EVN in November 2005 by Surcis et al. (2011). The big black arrows indicate the distance traveled by moving at a velocity of 10 km s⁻¹ over 20 yr. In the left panel, dotted contours and the dashed-line and black-yellow star have the same meaning as in Fig. 3.