You're aware that mercury is tidally locked, as is our moon, mars' moons, and others? I would call those real, physical effects, not negligible. As to running

You're aware that mercury is tidally locked, as is our moon, mars' moons, and others? I would call those real, physical effects, not negligible. As to running the numbers - agreed - but one can get an answer which is correct but incorrect if your premises are incorrect. At a galactic scale the perturbations are likely tiny, but a tiny force applied over enough time adds up. Anyway. It's a poor example that's almost certainly wrong but my point was more about how a single incorrect assumption can wreck a model.