apsidal motion(ap -să-dăl) Rotation of the line of apsides (i.e. the major axis) of an elliptical orbit so that the orbit is not closed and there is a gradual shift in the position of periapsis. It can occur if the gravitational field of a third body can produce a significant effect on a two-body situation, as with the Earth, Moon, and Sun, or if one or both stars of a binary system is appreciably distorted from a spherical shape.
apsidal motion[′ap·sə·dəl ′mō·shən]
The precession of the periastron of a binary system in the orbital plane of the two stars, resulting from tidal gravitational moments.