Top panel: variation of the observational phase shift with L for the azimuthal order m = 0 (solid line), m = 1 (dashed line), and m = 2 (dashed-dotted line). Bottom panel: variation of the observational phase shift with m at L = 1412. The error estimates are obtained from an average over m of the absolute value of the phase-shift difference between + m and − m, divided by . This estimation assumes that the error does not depend on m.