The plane of the Moon's orbit around the Earth is tilted by about 5° relative to the plane of of the ecliptic. The points where the orbit intersects the ecliptic are called the nodes. The northward-crossing one is known as the ascending node, whilst the one on the opposite side of the Earth, where the Moon crosses toward the south, is the descending node.

The line which connects them is called the node line. Eclipses can only occur when the node line aligns with the Sun.

Inconstant Moon