Speed of gravity propagation is said to be the same to speed of light. But is it true ? Something can't be found in thought experiments ? How about Sun and Mercury ? Solar system is said to move in the direction called sun facing point (on the celestial sphere). So position from which gravitational force of Sun acting Mercury will be the position some back in time. If so, this effect will slightly acuumlate (like free falling).
In Encyclopaedia Britannica 1969, there is a passage as follows (in item "Gravitation" ; original text). "If the action of gravitation were not absolutely instantaneous"(omission)"All experiments and observations were, however, consistent with the law, from the short distances employed in laboratory experiments to the long ranges used in interplanetary calculations". It says, action will be instantaneous.
