The lunar nodes: Head and Tail of the Dragon in astronomy and myth
What the lunar nodes are, why they regress in 18.6 years and how they became Rahu and Ketu in India and caput and cauda draconis in medieval Europe.
They are invisible points, without stars or light, and yet they have held a central place in the astrology of half the world. The lunar nodes are simply the intersections between the Moon's orbit and the ecliptic, but their link with eclipses turned them, in the imagination of many peoples, into monstrous creatures able to swallow the heavenly bodies.
Two crossings in the sky
Since the lunar orbit is tilted by about 5.1 degrees to the ecliptic, the Moon spends half the month north and half south of the Sun's apparent path. The ascending node is the point where the Moon crosses the ecliptic moving north, the descending node the point where it moves south. Eclipses occur only when the Sun and the Moon are aligned close to one of these two points.
The line of nodes is not fixed: because of the Sun's attraction it regresses along the zodiac, completing a full circuit in about 18.6 years. As a result the Sun meets the same node about every 346.6 days, an interval called the eclipse year. Each year therefore has two eclipse seasons, lasting just over a month and separated by about six months, which arrive some twenty days earlier every year.
Rahu and Ketu
In Indian tradition the nodes are called Rahu and Ketu. The myth, told in the Mahabharata and the Puranas, relates that during the churning of the ocean of milk the gods obtained amrita, the drink of immortality. A demon slipped in among them to take a sip, but the Sun and the Moon denounced him to Vishnu, who cut off his head with his discus. Since he had already drunk, the head became immortal under the name of Rahu, and the body became Ketu.
Ever since, according to the story, Rahu pursues the Sun and the Moon to take revenge and sometimes swallows them, but they soon reappear, because the head has no body and cannot hold them. In Indian astrology Rahu and Ketu are counted among the nine grahas, the celestial powers that according to this tradition influence human life, even though they are not visible bodies.
The dragon of the Latins
Through Arabic astronomy the image of the dragon reached medieval Europe. The ascending node was called caput draconis, the head of the dragon, and the descending node cauda draconis, the tail of the dragon. The symbols still used in almanacs today resemble a horseshoe, upright or upside down. Medieval astrologers attributed a favourable nature to the head and an adverse one to the tail, and took them into account in horoscopes as significant points.
The dragon as devourer of light appears elsewhere too. In China eclipses were explained, according to popular belief, as the work of a dragon or a celestial dog, and the imperial chronicles recorded them carefully because they were regarded as messages addressed to the ruler. The common motif is always the same: a monster hidden along the path of the Sun and the Moon.
A geometry turned into a story
In modern astrology the nodes have often been reinterpreted as symbols of direction and of inheritance from the past, a reading that does not belong to the ancient sources. Beyond interpretations, their history shows clearly how a purely geometrical concept, born to explain eclipses, became a mythical tale: the dragon that devours the light and is then forced to give it back.