The sky — astronomy

Lunar Nodes: Where the Moon’s Orbit Crosses the Ecliptic

The lunar nodes are the two points where the Moon’s orbit crosses the plane of the ecliptic, the Sun’s apparent path across the sky. The orbit is tilted by about 5 degrees, so most New and Full Moons miss the Sun–Earth line. An eclipse happens only when the lunar phase coincides with the Moon being near a node.

The tilted lunar orbit

The Moon circles the Earth roughly once a month, while the Earth and Moon together travel around the Sun. The Sun’s apparent yearly path across the sky is called the ecliptic, and it lies in the plane of Earth’s orbit. If the Moon’s orbit lay exactly in that plane, every New Moon would put the Moon in front of the Sun and every Full Moon would send it through Earth’s shadow. We would have eclipses every month.

In reality the Moon’s orbit is inclined to the ecliptic by about 5 degrees. That sounds small, but it is enough. The Sun and Moon each appear about half a degree wide, so in most months the New Moon passes above or below the Sun, and the Full Moon passes above or below Earth’s shadow. That is why eclipses are special events rather than monthly routine. You can read more about the cycle of phases in the article on lunar phases.

Ascending and descending nodes

Two tilted planes intersect along a straight line. The Moon’s orbital plane therefore cuts the ecliptic plane at two opposite points, called the lunar nodes (from the Latin nodus, meaning knot):

• The ascending node is where the Moon crosses from the south side of the ecliptic to the north side;
• The descending node is where it crosses from north to south.

Halfway between the nodes the Moon strays farthest from the ecliptic, about 5 degrees north and then south. The two nodes lie exactly opposite each other, so the line joining them, the line of nodes, passes through the centre of the Earth.

The time the Moon takes to return to the same node is the draconic month, about 27.2 days. It is slightly shorter than the sidereal month (about 27.3 days) because the nodes themselves shift, as described below.

Eclipse seasons

An eclipse needs two conditions at the same time:

1. The right phase: New Moon for a solar eclipse, or Full Moon for a lunar eclipse;
2. The Moon being close to one of the nodes, so that it lies almost on the ecliptic.

As the Sun moves along the ecliptic, it comes near the line of nodes twice a year, a little less than six months apart. These windows are called eclipse seasons. Each lasts a bit over a month, and during it a New Moon and a Full Moon can fall close enough to a node to produce eclipses. Usually two eclipses occur in a season, and sometimes three.

The Sun takes about 173.3 days to travel from one node to the next. Its return to the same node takes about 346.6 days, called the eclipse year, which is shorter than the calendar year. As a result, eclipse seasons arrive roughly 19 days earlier on the calendar each year.

NASA maintains detailed information and catalogues of past and upcoming eclipses, including maps showing where each can be seen.

The nodes drift backwards

The Moon’s orbit is not fixed in space. Mainly because of the Sun’s gravitational pull, the line of nodes slowly rotates in the direction opposite to the Moon’s own motion, that is, westward along the ecliptic. This is called the regression of the nodes. One full turn takes about 18.6 years, or roughly 19 degrees per year.

This is a real change in orbital geometry. It is not the same thing as the apparent retrograde motion of planets, which is a visual effect of our viewpoint on Earth.

The 18.6-year cycle has visible consequences. It affects how high or low the Moon rises and sets during different periods, and, through its effect on the draconic month, it also underlies the Saros cycle (about 18 years and 11 days), after which eclipses repeat with similar geometry.

Astronomers also distinguish the mean node, which moves smoothly, from the true (osculating) node, which includes small perturbations in the orbit. They differ by less than two degrees: the mean node is the averaged position of the line of nodes, and the true node is its momentary position.

The nodes in astrological tradition

This is a cultural tradition, not an astronomical fact. In Western astrological tradition the nodes are called the North and South Node and are linked with themes of direction in personal growth and of the familiar ground a person starts from. In the Indian (Vedic) tradition they are known as Rahu and Ketu, and in older European texts as the head and tail of the dragon. The connection is historical: ancient observers noticed that eclipses happen near these points and imagined a dragon swallowing the Sun or Moon.

An astrological reading is a prompt for reflection and inspiration, not proof that the nodes influence our lives. You can find more on the symbolism in the article on the lunar nodes in astrology.

In short: a tilt of about 5 degrees is why eclipses are rare treats rather than monthly events, and the nodes are the points that decide when the sky offers us one.

Positions in Astronativa are astronomically exact. The interpretation is for inspiration. · Glossary · Terms · Privacy