Tilt angle calculation. Panel a shows an example of the tilt angle calculation for the coronal hole on April 30, 2021, at 23:50. The red crosses indicate the latitudinal geometric centers of mass, which are then fitted (red line). The angle between the fit and the north–south direction (blue line) is defined as the tilt angle of the coronal hole. In panel b a schematic of the tilt angle calculation solely caused by differential rotation is shown. The light blue arrows highlight the solar differential rotation rates at different latitudes (not to scale).