in Astronomy, the two points where the orbit of a planet intersects the ecliptic.

Such are the two points C and D, fig. 1. of which the node C, where the planet ascends northward above the plane of the ecliptic, is called the ascending node, or the dragon's head, and is marked thus Q. The other node D, where the planet descends to the south, is called the descending node, or the dragon's tail, marked thus S.

The line CD, wherein the two circles CEDF and CGDH intersect, is called the line of nodes. It appears from observation, that the line of the nodes of all the planets constantly changes its place, and shifts its situation from east to west, contrary to the order of the signs; and that the line of the moon's nodes, by a retrograde motion, finishes its circulation in the compass of 19 years; after which time, either of the nodes having receded from any point of the ecliptic, returns to the same again; and when the moon is in the node, she is also seen in the ecliptic. If the line of nodes were immovable, that is, if it had no other motion than that whereby it is carried round the sun, it would always look to the same point of the ecliptic, or would keep parallel to itself, as the axis of the earth does.

From what hath been said, it is evident, that the moon can never be observed precisely in the ecliptic, but twice in every period; that is, when she enters the nodes. When she is at her greatest distance from the nodes, viz. in the points E, F, she is said to be in her limits.

The moon must be in or near one of the nodes, when there is an eclipse of the sun or moon.

To make the foregoing account of the motion of the moon's nodes still clearer, let the plane of fig. 2. represent that of the ecliptic, S the sun, T the centre of the earth, L the moon in her orbit DN dn. NN is the line of the nodes passing between the quadrature Q and the moon's place L, in her last quarter. Let now LP, or any part LS, represent the excess of the sun's action at T; and this being resolved into the force LR, perpendicular to the plane of the moon's orbit, and PR parallel to it, it is the former only that has any effect to alter the position of the orbit, and in this it is wholly exerted. Its effect is twofold: 1. It diminishes its inclination by a motion which we may conceive as performed round the diameter D d, to which LT is perpendicular. 2. Being compounded with the moon's tangential motion at L, it gives it an intermediate direction Lt, through which and the centre a plane being drawn, must meet the ecliptic nearer the conjunction C than before.