  Many helioseismic measurements suffer from substantial systematic errors. A
particularly frustrating one is that time-distance measurements suffer from a
large center to limb effect which looks very similar to the finite light travel
time, except that the magnitude depends on the observable used and can have the
opposite sign Zhao et al. (2012). This has frustrated attempts to determine the
deep meridional flow in the solar convection zone, with Zhao et al. (2012)
applying an ad hoc correction with little physical basis to correct the data.
In this letter we propose that part of this effect can be explained by the
highly asymmetrical nature of the solar granulation which results in what
appears to the oscillation modes as a net radial flow, thereby imparting a
phase shift on the modes as a function of observing height and thus
heliocentric angle.
