advance of perihelion

advance of perihelion A gradual turning of the major axis of an orbit in the same direction as the body moves along the orbit; also known as apsidal motion. An advance of perihelion means that the longitude of perihelion of the orbit increases. In the Solar System, the effect is caused mostly by the gravitational attractions of the planets. A similar effect, the advance of pericentre, is seen in the orbits of binary stars, caused by the oblateness of the stars themselves. In the 19th century, a small unexplained advance of Mercury's perihelion of some 43” per century was attributed to an undiscovered planet within the orbit of Mercury (see Vulcan). This effect is now known to be caused by the curvature of space near the Sun, as predicted by the general theory of relativity.