Lunar node

The lunar nodes are the orbital nodes of the Moon, that is, the points where the orbit of the Moon crosses the ecliptic. The ascending node is where the moon crosses from south of the ecliptic to north of the ecliptic. The descending node is where it crosses from north of the ecliptic to south of the ecliptic.

Eclipses occur only near the lunar nodes: Solar eclipses occur when the passage of the Moon through a node coincides with the new moon; lunar eclipses occur when passage coincides with the full moon. A lunar eclipse may occur if there is a full moon within 11° 38' (Celestial Longitude), of a node, and a solar eclipse may occur if there is a new moon within 17° 25' of a node.

The plane of the lunar orbit precesses in space and hence the lunar nodes precess around the ecliptic, completing a revolution (called a draconic or nodal period, the period of nutation) in 6798.3835 days or 18.612958 years (note that this is not the same length as a saros). The same cycle, measured against an inertial frame of reference such as ICRS (relative to the stars) is 18.599525 years.