It has long been said, but not until now have I heard why Newton got it wrong, but only slightly.

Basically Gravity acts with an inverse law. Simple enough, but the planets beg to differ.

However Einstein's Relativity states the time changes with speed. And Gravity comes out of the Space-Time thingy.

Planets (etc) are traveling at a speed. Low enough for most purposes but enough to need an added term to Newton's Law of Gravity. Small but cumulative when predicting the position of planets.

So There you have it, if I am right. Newton was wrong - slightly.