Sketch illustrating pebble accretion in two different frames. a) Local frame: the co-moving frame is centred on the planet where the x-axis is pointing radially away from the star and the y-axis follows the orbital direction of the planet. Grey arrows purely indicate the gas velocity of the shearing flow, and the blue arrow shows the trajectory of the pebble, with the x-axis impact distance b and the y-axis velocity v_y(b). b) Global frame: the planet orbits around the central star with a semi-major axis a_p. The black arrow depicts the planet motion and the blue arrows illustrate the trajectories of pebbles that cross the orbit of the planet.