Perihelion Precession of Mercury

In the 19th century, astronomers observed that Mercury’s elliptical orbit was not stationary. Its point of closest approach to the Sun, the perihelion, was slowly advancing, or precessing. While most of this precession was explained by the gravitational tugs of other planets according to Newton’s laws, a small discrepancy of about 43 arcseconds per century remained unaccounted for. This anomaly puzzled scientists, with some proposing the existence of an undiscovered planet, Vulcan, between Mercury and the Sun.

In 1915, Albert Einstein applied his new theory of general relativity to the problem. His calculations showed that the curvature of spacetime caused by the Sun’s mass would introduce a correction to the Newtonian description of gravity. This correction perfectly accounted for the missing 43 arcseconds per century without any ad-hoc parameters. Unlike Newton’s theory, where orbits are closed ellipses (in a two-body system), general relativity predicts that orbits are not closed but trace a rosette pattern. This effect is most pronounced for objects in strong gravitational fields and with eccentric orbits, making Mercury the ideal candidate in our solar system. The successful explanation of Mercury’s perihelion precession was one of the first strong pieces of evidence that general relativity was a more accurate description of gravity than Newton’s theory.