I don't believe the planets act on us in a 3rd dimensional, physical way. They act on us from a higher sphere of consciousness and existence, of which quantum physics is barely beginning to scratch the surface. Time and space do not exist there, thus how big the planet or body or how far away it is physically is irrelevant. What exactly is the mechanism, I have no idea, nor does anyone else, I suspect. If anyone asks how can the gravitational pull of a planet billions of miles way affect our lives and I say that it doesn't. THAT really confuses people when they ask about astrology.
Again, I essentially agree with MDinaz as I usually do. But in explaining astrology, this would be an explanation to an esotericist in training. To the general public, from unfortunately a lot of personal experience early in my adult life, these kinds of explanations are just gobble-de-gook. It takes too much to even begin to explain it.
So I just go over the signs on the road or signs of the forest trail explanation. Then I don't run into the ugly head of scientism attacking my credibility. Further I just say that it represents a part of the Spiritual World View rather than the Materialistic World View(all that is, is made of matter that can be observed with eyes, microscopes, telescopes, and particle colliders).