Answer:
One of the indirect proofs that orbits change is actually in the growth of our own teeth when we were children. our teeth are some of the most basic, and primitive
parts of our bodies. They grow on a 9 day cycle, which was an ancient full moon to full moon cycle when the Earth and the Moon were a lot smaller, and closer together, and the co-orbital period was only 9 days, not the 29.5 days that it is currently.
So Given any two " Planets " that co-orbit a common gravitational center, the larger planet will grow larger far faster than the smaller planet, and the larger planet will accelerate the smaller planet to a higher orbit with a longer period, and decelerate itself to a lower orbit with a longer period, and the absolute value of the center to center distance will increase, and the orbital period will increase. The two orbs and their common gravitational center will remain co-linear through out the gradual growing and changing process.
This is an important process for the enlargement of the solar system as time passes, and an important process for larger galaxies as they attract and merge with smaller galaxies.
All of the planets grow larger at an accelerating rate, and thus systems spiral outward concentrating mass into larger and fewer galaxies, solar systems, and planet - moon systems.