Precession of the Perihelion of Mercury

The orientation of Mercury's orbit is found to precess in space over time, as indicated in the adjacent figure (the magnitude of the effect and the eccentricity of Mercury's orbit are greatly exaggerated for purposes of illustration). This is commonly called the precession of the perihelion, because it causes the perihelion (point of closest approach of the planet to the Sun) to move around the center of mass. Because of the effects of general relativity, on each orbit the planet fails to return exactly to the starting point of the orbit. The net effect is approximately as if Mercury were on an elliptical orbit (as predicted by Kepler) but with the axis of the ellipse slowly rotating in space.

Precession of Orbits

The precessional effect is present for all orbits. However, it is largest in the Solar System for Mercury because it is the planet that feels the Sun's gravity the strongest and also moves the fastest on its orbit.

The most precise test of the precessional prediction now comes from the Binary Pulsar system, which is two neutron stars in orbit around each other. It has as much orbital precession in one day as Mercury does in a century (the orbit of the Binary Pulsar precesses by 4.2 degrees per year). Observations on the Binary Pulsar confirm the relativity prediction with high precision.

Only part of this motion can be accounted for by perturbations in Newton's theory. There is an extra 43 seconds of arc per century in this precession that is predicted by general relativity and observed (recall that a second of arc is only 1/3600 of an angular degree!). This effect is extremely small, but the measurements are precise and can detect such small effects very well. Thus, the precession of Mercury's perihelion is evidence favoring the general theory of relativity over Newton's theory.

Animation: precession of the perihelion of Mercury