How then does the direction of Earth acceleration compare with the direction of the visible Sun? By direct calculation from geometric ephemerides fitted to such observations, such as those published by the U.S. Naval Observatory or the Development Ephemerides of the Jet Propulsion Laboratory, the Earth accelerates toward a point 20 arc seconds in front of the visible Sun, where the Sun will appear to be in 8.3 minutes. In other words, the acceleration now is toward the true, instantaneous direction of the Sun now, and is not parallel to the direction of the arriving solar photons now. This is additional evidence that forces from electromagnetic radiation pressure and from gravity do not have the same propagation speed.

Abstract.  Gravity has no aberration, and propagation delays cannot be used without destroying angular momentum conservation at an unacceptable rate.  Even the curved spacetime explanation (“gravity is just geometry”) breaks down when masses and speeds are large, as in binary pulsars.  But if gravity or spacetime curvature information is carried by classical propagating particles or waves, a modern Laplace experiment places a lower limit on their speed of 1010 c.  The so-called Lorentzian modification of special relativity permits such speeds without need of tachyons.  But there are other consequences.  If ordinary gravity is carried by particles with finite collision cross-section, such collisions would progressively diminish its inverse square character.  Gravity would gradually convert to inverse linear behavior on the largest scales.  Curiously, at all scales greater than about 2 kiloparsecs, gravity can be modeled without need for dark matter by an inverse linear law.  The orbital motions of Mercury and Earth may also show traces of this effect.  Moreover, if gravity were carried by particles, a collapsed ultra-dense mass between two bodies could shield each of them from the gravity of the other.  Anomalies are seen in the motions of certain artificial Earth satellites during eclipse seasons that behave like shielding of the Sun’s gravity.  Certain types of radiation pressure might cause a similar behavior, but require far more free parameters to model.  Each of these effects of particle-gravity models has the potential to lead to a breakthrough in our post-Einsteinian understanding of gravitation.  This would also change our views of the nature of time in relativity theory.