The method to determine the moving direction of a NP (black dot at the original point). Panels (a) and (b) show the surfaces γ 0 r + λ γ 1 r = 0 , γ 0 i + λ γ 1 i = 0 and their normal directions, respectively. Panel (c) shows that the tangential direction of the NL (with red color) is perpendicular to the above two normal directions. (d) The projection of the NL onto the λ = 0 plan is the trajectory of the NP and the tangential direction is the moving direction.