Dependence of the reference frame on magnitude. The diagram shows the spin components as in Fig. 3, but subdivided by magnitude. The points at the faint end (G ≳ 15) are estimated from the proper motions of quasars. At the bright end (G ≲ 13), the spin is estimated from the differences in stellar proper motions between Gaia DR2 and the HIPPARCOS subset of TGAS in Gaia DR1.