Positions of the components NE and NE′ (Blue dots), and positions of the radio core, the components C1, C2, and C3 for all epochs (red dots). We define the dotted blue line (the least square line of the blue dots) as the eastern limb, and the dotted red line (that of the red dots) as the western limb. The intersection of the extrapolated lines (shown as the dotted grey lines) is labeled A. The position of the core is set to (0, 0).