NODE. An astronoinieal term used in con nection with planetary orbits. If we disregard certain small perturbations (q.v.) of planetary motion, we may say that each planet 1,1°N-es in an elliptic curve. having the sun in one of its foci. This curve lies in a plane called the plane of the orbit : and in the ease of the earth this orbital plane is called the plain of the eclip tic. Now, if these orbital planes are imag ined extended to the celestial sphere, they will cut out upon it great cireles, one correspond ing to each orbital plane. Such a great cir cle cut out by the plane of the earth's orbit is called the ecliptic (q.v.). If we consider the ecliptic and the corresponding great circle belonging to any other planet. we shall find that they intersect at two points on opposite sides of the celestial sphere. These two point.. in the l'a..t• of any planet other than the earth. are called the nodes of its orbit.
Thus all the planetary nodes lie on the ecliptic circle: and the position of any node is given by stating its celestial longitude. (Sec LATITUDE
A NI) This 10101 Ude of the node is one of the elements (q.v.) of a planet's orbit. Niure the two nodes are directly opposite one another, their longitudes must differ by exactly so that the longitude of the one beMg given, that of the other is known also. 11y com mon consent. astronomers select that one of the nodes whose longitude is to lie given as an 4-lentent of the orbit as the USN y node. This is the node corresponding to the pas-age of the planet from the south side of the ecliptic to the north side in the course of its orbital revolution around the sun. The other node is Palled the Ile sr, :lain!, node.
The effect of planetary perturbations (q.v.) is to emlli Se the nodes to 1110Ve backward on the ecliptic. The rate of motion is very slow. t he being in the case of Uranus, whose node will traNel once around the ecliptic in 37.000 years, while that of Mercury will I 66, Onfl year, in a single revolution.