Venus' non-relativistic contribution to Mercury's precession

[Eugene]

TL;DR Summary

why does the orbit rotate as oppose to not rotate and just change its shape

Mercury's orbit is rather eccentric, its perigee brings it much nearer to the sun than its apogee. Venus by contrast has practically a circular orbit. When Mercury is approaching apogee, it can get close to venus if it is in the similar part of its orbit. At this point, Mercury's precession advances the most. but why does this venus gravitational pull cause mercury's orbit to rotate(its elliptical orbit maintains the same shape) as opposed to changing (elongating) Mercury's orbit?

 

[lippyka]

The gravitational pull of Venus does not cause Mercury's orbit to rotate (in the sense of changing its orientation). It causes the perihelion of Mercury—the point at which it is closest to the Sun—to precess, or gradually shift in the direction of motion. The precession of Mercury's perihelion is caused by the combined gravitational pull of all the other planets, but Venus has the strongest effect due to its proximity to Mercury.The elliptical shape of Mercury's orbit is determined primarily by the mass of the Sun and the distance between Mercury and the Sun. The gravitational pull of Venus will not change this shape, but it will cause the perihelion to precess. This precession causes Mercury to move slightly faster when it is closer to the Sun, and slightly slower when it is farther away, resulting in a slight change in its orbital period.