Good... Lord. Measurements of orbital path deviations are refined enough to capture deviations that tiny. Just for reference, if the average diameter of the moon is 3475 kilometers, or 3 475 000 000 millimeters, we are calculating deviations 0.0000001% in size. HOLY MOLY. I just figured that orbital paths had more annual (or monthly in the case of the moon) deviation than that. I guess that means 4+ billion years of astronomical and gravitational activity in our solar system have resulted in very uniform orbits.Tidal forces also affect the shape of the lunar orbit (specifically its eccentricity) over time as well. But even after tidal forces are taken into account, there is an extra bit that remains unaccounted for. It seems the difference between the Moon’s apogee and perigee is changing by an extra 3.5 millimeters per year, and we’re not entirely sure why that is.