The ascending node is the point in an orbit where a body traveling from south to north crosses a reference plane, such as the plane of the ecliptic (in the case of a Solar System object) or the celestial equator. The opposite point in the orbit, where the body moves from north to south across the reference plane, is the descending node. The longitude of the ascending node is an orbital element.

 

 

Descending node

The descending node is the point in an orbit where the orbiting body crosses a reference plane, such as the ecliptic or the celestial equator, going from north to south.