Question: What's the difference between the predictions that general relativity made and gravity made about Mercury? Why was the general relativity prediction correct?

Gabriel Balaban answered on 16 Mar 2018:

Mercury does not move in the way that the physics before Einstein would predict. When you add the curved space-time of general relativity into the calculations of Mercury’s orbit then the prediction is more accurate. This is one reason why people began to believe in the general relativity theory.

The reason that general relativity theory was better than classical physics is because it included more of the things that affect Mercury’s motion,