The Moon’s nodes

What the nodes are

The Moon’s orbit is tilted about five degrees to the path the Sun takes through the zodiac. Twice a month the Moon crosses that path: going north at one node and south at the other. The two crossing points always sit exactly opposite each other, and the pull of the Sun turns the whole orbit slowly backwards, so the pair travels back through the signs, about a degree and a half a month and once round in eighteen and a half years.

Counting the drift

One full turn takes 18.6 years, which is about 19° a year and a little over three minutes of arc a day. Twelve signs in 18.6 years gives roughly eighteen and a half months for each pair of signs. Say the north node stood at 10° Aries when someone was born, with the south node at 10° Libra. A year later the north node has slipped back 19° to about 21° Pisces: backwards, so from Aries into Pisces and not into Taurus. After a little over nine years it has gone half the circle and stands at 10° Libra, on the birth south node, the half return. Because the two nodes are always opposite, there are only six axes to be born under: Aries and Libra, Taurus and Scorpio, and so on round.

Why eclipses fall near them

A new moon or a full moon only lines up closely enough with the Sun and the Earth to throw a shadow when it happens near a node. That is why eclipses come in seasons about six months apart, and why the eclipses of any one stretch fall in the pair of signs the nodes are passing through. The eclipses page draws the same drift from the other side.

True and mean

There are two ways to give the position. The mean node is a smooth average that only ever goes backwards. The true node is where the orbit actually crosses at that minute: it swings a degree and a half or so either side of the mean, and every few months stands still and runs forward for a while. The wheel and the readings use the true node. The dates the axis changes sign use the mean node, because the true node rocks back and forth over a boundary for weeks and would give a string of dates for a single change. Charts made elsewhere use either, so a difference of a degree or two is usually that.

How astrology uses them

The north node is traditionally read as the direction of growth, what is unfamiliar and asks for effort, and the south node as what comes easily and is already known. A nodal return, when the nodes come back to where they were at a birth, falls at about eighteen and a half, thirty-seven, fifty-six and seventy-four, and is treated as a marker in a life rather than an event. Halfway between, at about nine, twenty-eight, forty-six and sixty-five, the nodes stand opposite their birth positions, which is read as a half return. That is the vocabulary of the tradition, described here; nothing on this page predicts anything.

Older names, older readings

Medieval astrologers called the pair the Dragon’s Head and the Dragon’s Tail, Caput and Cauda Draconis, after the old picture of a dragon that swallows the Sun or Moon at an eclipse. In Indian astrology they are Rahu and Ketu and are counted among the nine grahas beside the Sun, the Moon and the five visible planets. The reading was simpler then: the head was said to increase whatever it touched and the tail to decrease it. The language of growth and habit, and of past lives in some schools, belongs mostly to the twentieth century.

What the nodes are mixed up with

The 18.6 years of the nodes is not the saros, the cycle of eighteen years and eleven days after which a similar eclipse comes round; the two numbers sit close and measure different things. The same swing of the orbit does show in the sky without any chart: when the north node is at the start of Aries the Moon ranges furthest north and south over a month, to about 28.6° from the celestial equator, and some nine years later, with the node at the start of Libra, only to about 18.3°. The degree the Moon itself holds at a birth is a separate matter, on the Moon sign page; the houses the nodes fall in are explained under the houses, and the full wheel is the birth chart.