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 s. to the n. side of the ecliptic being called the ascending node (c2), and the other the descending node (U). 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 line drawn joining these two points is called the line of nodes of each body. It is scarcely necessary to add that, as 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 west ward). This mption is produced by the mutual attractions of the planets, which tend to draw each of Meth. oUt of.the:plane'of 'its orbit; add it depends upon the relative posi tions 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 favor of a retrograde motioa; and we find observation that, in an average of many revolutions round the sun, a constant retrogra dation of the node takes place. The determination of this retrogradation in the ease of the planets is a most complicated problem, as the separate action of each on the others has to be taken into account; but in the case of the moon's nodes, the immensely pre ponderating attraction of the earth, and its great relative magnitude as compared with the moon, enable us to throw out of account any other disturbing influence, and at the same time to exhibit clearly the cause of this motion of the nodes. 'Suppose the moon
to have attained her greatest n. latitude, and to be descending towards the ecliptic, and the earth to be in longitude e. ude between her and her previous deSeendin node, then the earth's attraction will tend to depress the moon's orbit, and cause her to descend to the plane of the ecliptic sooner than she would otherwise have done; in (this case we have a retrogra dation of the node. Again, supposing the moon placed as before, but the earth in advance of the line of nodes, then the earth's attraction will tend to draw the moon forward in her orbit so as to meet the ecliptic in a point beyond the previous descending. node; in this case, the moon's node has advanced. As in the case of the planets, however, the retrograding tendency preponderates. The average annual retrogradation of the nodes is very small in the case of the planets, but considerable in that of the moon. See Moox. Iu calculating the courses of the planets, the " length " of the ascending node, or its distance in longitude from the vernal equinox, is a most important element. See