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Why does Mercury’s orbit shift slightly over time?

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Brian Greene sits in a living room, holding a pen, with handwritten equations m = E/c^2 and E = mc^2 on a whiteboard beside him, highlighting c^2 as the speed of light. — relativity Brian Greene kicking off his *Your Daily Equation* video series with Albert Einstein's famous equation E = mc².

Mercury’s orbit shifts, or precesses, because of the curvature of space-time caused by the Sun’s immense mass.

While most of this orbital motion can be explained by the gravitational influence of other planets using Isaac Newton’s laws, a small discrepancy of 43 arc seconds per century remained unexplained for years. In 1915, Albert Einstein demonstrated that his general theory of relativity accounted for this shift precisely.12

In Einstein’s framework, gravity is not a force acting across space, but a geometric property where matter tells space-time how to curve, and space-time tells matter how to move. Because Mercury is the closest planet to the Sun, it moves through a region where the curvature of space-time is more pronounced, causing its elliptical orbit to rotate (precess) in a way that Newtonian physics alone could not predict. This successful calculation served as a landmark observational verification of general relativity.3412