What are the lunar nodes, and why do eclipses happen there?

What are the lunar nodes?

The Moon’s orbit is tilted about 5° to the ecliptic, so for half of each month the Moon is above the Sun’s path and for half below. The two points where it crosses are the nodes: the ascending (North) node, where it crosses going north, and the descending (South) node, directly opposite. They are always exactly 180° apart.

They are points, not objects — there is nothing physically at a node. Because the Moon’s orbit itself slowly rotates, the nodes drift steadily backwards around the zodiac, completing a circuit in about 18.6 years.

Mean node and true node

The Moon’s orbit wobbles, so the nodes can be given two ways. The true node is the exact, instantaneous crossing point, which drifts backwards but briefly jitters forwards; the mean node is that motion averaged into a smooth, always-backward drift. They differ by up to about 1.5°.

OrbHeaven uses the mean node — the older convention — and says so. Neither is more “correct”; they answer slightly different questions, and the difference is small enough that it rarely changes a sign.

The nodes and eclipses

Most New Moons pass above or below the Sun, and most Full Moons above or below the Earth’s shadow, because of that 5° tilt. Only when a New Moon or Full Moon happens near a node do the three bodies line up closely enough for an eclipse — a solar eclipse at the New Moon, a lunar eclipse at the Full.

This is why eclipses come in seasons about six months apart, when the Sun is near the node axis. OrbHeaven does not compute or predict eclipses; it locates the nodes and the lunations, and this page explains the geometry rather than forecasting any particular eclipse.

Source: NASA/GRC/Jordan Cochran · composite · U.S. public domain (courtesy NASA; no endorsement implied).

Rahu, Ketu, and the dragon

The nodes are old and were vivid long before their geometry was understood. In Indian astrology they are Rahu (north) and Ketu (south), treated almost as shadow-planets; in the medieval West they were the dragon’s head and tail — caput and cauda draconis — the creature imagined to swallow the Sun at an eclipse.

Predicting where the nodes would be, and so when eclipses could occur, drove some of the oldest exact astronomy: the Babylonian Saros cycle of about 18 years already let eclipses be anticipated thousands of years ago.

What the nodes are read as

Interpretation, labelled as such: the tradition reads the North Node as a direction of growth — the unfamiliar thing worth reaching for — and the South Node as what already comes easily and can be leaned on less. They are an axis, always read as a pair.