Lunar Node

A lunar node is one of two points where the Moon's orbital path around Earth crosses the ecliptic, the plane of Earth's own orbit around the Sun. Because the Moon's orbit is tilted by about 5 degrees relative to the ecliptic rather than lying perfectly flat within it, the Moon spends roughly half of each orbit slightly above the ecliptic plane and half slightly below it, crossing through it at exactly two points: the ascending node, where the Moon crosses moving from south to north, and the descending node, where it crosses moving from north to south. Lunar nodes matter enormously for eclipses: a solar or lunar eclipse can only occur when a new moon or full moon happens to fall very close to one of these two node points at the same time, which is precisely why eclipses don't happen every single month despite the Moon passing between Earth and Sun, or into Earth's shadow, on a monthly basis. The nodes themselves are not fixed in space; due to gravitational tugging mainly from the Sun, they slowly rotate backward around the ecliptic in a cycle lasting about 18.6 years, called the nodal precession cycle, which is also why the pattern and visibility of eclipses shifts gradually over the decades.