We know, objects near the earth Experience an Accelerating Force. The force averages 32 feet per second, per second. But, when your analogy no longer explains good, repeated observations, it is time to move along. They also know that there was practically no serious experimentation between Newton and the present day. Results have come sooner than anyone expected.
Mercury moves quickly around the sun in an elliptical path, like a horse running around a racetrack.
But now Dicke has found that the sun is not spherical It has a slight bulge around the equator. This bulge also helps move the racetrack around. The unexplained error is now about 39 of arc. A small amount, perhaps, but larger than the limits of our observations and computing abilities. A new theory of gravity must now be sought.