Final answer:
Einstein's explanation for Mercury's perihelion advance is rooted in general relativity, which accounts for the curvature of spacetime around the Sun, explaining the observed discrepancy not accounted for by Newtonian gravity.
Step-by-step explanation:
Einstein suggested that the regular change in the perihelion of the planet Mercury could be explained by the effects of general relativity. This theory predicts that due to the curvature of spacetime around the Sun caused by its mass, the perihelion of Mercury should advance slightly more than predicted by Newtonian gravity. Gravity fields and the resulting distortion of spacetime were the subjects of intense research and had already been confirmed in measurements of Mercury's orbit prior to Einstein's theory becoming accepted.
According to Newtonian gravitation, the expected advance of Mercury's perihelion, due to the gravitational effects of other planets, would be about 531 arcsec per century. Yet, it was observed to be 574 arcsec per century, indicating a discrepancy. Einstein's general relativity filled this gap accurately, predicting an additional 43 arcsec per century, which aligned closely with the observed values. This phenomenon, often seen in systems with intense gravitational fields, was one of the early successes of Einstein's theory.