advance of perihelion
Perihelion advance of a planet's orbit, shown exaggerated.
The advance of perihelion is the slow rotation of the major axis of a planet's orbit in the same direction as the revolution of the planet itself, due mainly to gravitational interactions with other planets. A small additional advance of the perihelion of Mercury, by 43 arcseconds per century, was eventually explained as an effect of the general theory of relativity.
In the case of close binary stars, the advance of pericenter may additionally be caused by mass transfer and the stars' distorted (elliptical) shapes. Advance of perihelion (or pericenter) is also known as apsidal motion.