Nodes

Nodes, in Astronomy, are the two points in which the orbit of a planet intersects the plane of the ecliptic, the one through which the planet passes from the south to the north side of the ecliptic being called the ascending node ($\backslash Omega$), and the other the descending node ($\backslash varpi$). As all the bodies of the solar system, whether planets or comets, move in orbits variously inclined to the ecliptic, the orbit of each possesses two nodes, and a straight line drawn joining these two points is called the line of nodes of each body. It is scarcely necessary to add that as the earth moves in the plane of the ecliptic she has no nodes. The places of the nodes are not fixed points on the plane of the ecliptic, but are in a constant state of fluctuation, sometimes advancing (eastward), and at other times receding (moving westward). This motion is produced by the mutual attractions of the planets, which tend to draw each of them out of the plane of its orbit; and it depends upon the relative positions of the planets with respect to another planet whether that planet's nodes shall advance or recede. On the whole, however, the majority of possible 'relative positions,' or configurations, as they are called, is in favour of a retrograde motion; and we find by observation that in an average of many revolutions round the sun a constant retrogradation of the node takes place. The determination of this retrogradation in the case of the planets is a most complicated problem, as the separate action of each on the others has to be taken into account. The revolutions of the planetary nodes are accomplished very slowly, never amounting to as much as a single degree in a century. The nodes of the lunar orbit retrograde with much greater speed under the disturbing influence of the sun. It is owing to the fact that they complete a revolution in nearly eighteen Julian years and eleven days that series of eclipses regularly recur in that period. See ECLIPSES, ORBIT, PERTURBATIONS; and Herschel's Outlines of Astronomy.