Sorry to bother you, but I did not get anywhere answer what exactly moves Mercury periapsis. "Sun gravity" or "GR" or "warp of spacetime" are very broad answers, I want to know how they affect this guy. As far as I learn, at first it seems that root of such behavior is speed of gravity, so when Mercury goes away from the Sun, gravity needs more time to travel and thus trajectory somewhat changes. But if so, there must be the reversed effect when Mercury moves closer. If these effects are not equal by absolute value, why?
My second thought was the "descending" path along gravity curvature is not symmetrical to "ascending". Then question is "Why?" again.
If shorter, if there is a difference in Newton's and Einstein's formulas, what's this difference means if said with common words?