in astronomy, the line along which the orbital plane of a celestial body intersects the principal plane passing through the primary. The plane of the ecliptic is taken as the principal plane in the study of the motion of the planets, comets, and other bodies of the solar system, and the equatorial plane of the earth is usually taken as the principal plane in the study of the motion of the moon and artificial earth satellites. The points of intersection of the line of nodes with the celestial sphere, whose center is at the primary, are called the nodes of the orbit.