The Nodes of the Moon
In astrology, the Moon's Nodes (the Dragon's Head and Tail) are not planets in the strict astronomical sense, but rather sensitive points on the ecliptic, where the pathway of the Moon crosses the course of the Sun. The ecliptic is the path of the Sun through the heavens as seen from the Earth and projected onto the Celestial Sphere; the body of the "energy dragon" is the fourth-dimensional path of the Moon by declination, as she weaves her web around the earth.
The nodes are also known as the Dragon's Head (Caput Draconis), the North Node, and Dragon's Tail (Cauda Draconis), the South Node, although in Indian Astrology they are called Rahu and Ketu. In Mythology, this Dragon sought to devour the Sun and Moon, so causing the eclipses. The North Node, the Dragon's Head, is called the ascending node, because it marks the point where the Moon crosses the ecliptic from southern to northern celestial latitudes, and the South Node is called the descending node, marking the descent from northern to southern latitudes. The North and South Nodes are an axis, always in perfect opposition to each other, so the signs are affected in pairs (Aries/Libra; Pisces/Virgo; Aquarius/Leo etc). All the planets have nodes except the Sun and the Earth, but the Moon's Nodes are by far the most significant, in terms of human destiny.
In Western Astrology, the Dragon's Head, a "shadow planet" exalted in Gemini, is considered generally benefic (good), whilst the Dragon's Tail, exalted in Sagittarius, is considered malefic (evil) in influence. Contrary to some ill-informed comments I have recently noticed on the WWW, Indian (Vedic) Astrology considers both nodes to be markedly unfortunate, due to their karmic, instinctive and unconscious nature, with Rahu being the worse. After all, in the pursuit of moksha (liberation), "good" karma is just as binding as "bad" karma—and more seductive.
The motion of the Moon's Mean Node is retrograde through the zodiac at the rate of roughly one degree every 19 days (compare the 19 year cycle of lunation). I should clarify here that the Moon's Mean Node is always retrograde, but the True Node oscillates according to the Moon's "wobble". This is caused by the gravitational effects of the Earth/Moon interaction, so the True Node sometimes has periods of direct motion. These periods of direct motion are considered unfortunate, as they go against the "natural motion" of the nodes. The Mean Node averages this "wobble" out, so giving a clearer picture.
The placement of the Nodes illustrates a relationship between the Moon, the Earth and the Sun. The Moon's Nodes mark the points where the path of the Moon's orbit around the Earth crosses the path of the ecliptic (the apparent path of the Sun and planets around the Earth) as viewed from the surface of the Earth. An eclipse takes place when either a New Moon (producing a Solar Eclipse) or a Full Moon (producing a Lunar Eclipse) occurs close to the either of the Nodes. As ancient astrologer-priests discovered, Eclipses can thus be calculated by observing the movement of the nodal axis, shown by Dwight Ennis in Eclipses and the Moon's Nodes.
The significance of the Nodes has been widely underestimated in Western Astrology over the past fifty years or so, due to some extent to the rise of "psychological" or "humanistic" interpretative paradigms that have devalued many traditional features of the astrologer's toolkit. However, traditional Western astrology along with Vedic, or Indian astrology, places the Nodes on the same critical level of importance as the Sun, Moon and other visible planets. There is no doubt that this elevation of the Nodes to planetary status is justified, for experience shows that the movement of the Nodes has a very significant effect on human life and on the destinies of all beings who dwell on the surface of the Earth.
Progressions and transits to the natal Nodes invariably signify major releases of energy, related to the planet in question and the house activated thereby. The transiting nodes also have a remarkably powerful effect whenever they pass over a natal planet or other chart factor, such as the Part of Fortune (material wellbeing), the ascendant/descendant cusps (self vs other) or the MC/IC (public/career vs private/domestic). The Dragon's Tail in particular seems to mark relationships with a genuinely karmic quality, often signifying the emergence (and sometimes finalising) of commitments from the deep past. The Dragon's Head is more concerned with activating or energising personal ambitions, aims and developing purposes.
The degree in which the North Node is placed in the natal chart is a highly significant one. For example, if your Dragon's Head is placed in the 22nd degree of Aries, the 22nd degree of each of the other zodiacal signs is called the nodal degree. The South Node of course is placed in the opposing degree, but this nodal degree is also considered a very sensitive point in each of the signs, so whenever the nodal degree is activated by progression or transit, significant karmic events may be expected. Moreover, should any natal planet be placed in a nodal degree, it's influence is also strengthened for good or ill, depending on its character and condition.
The best way to understand the astrological nature of the Nodes might be to envisage them as bubbling pools of karmic energy. These have a massive potency, which is associated with powerful events and feelings whenever they are activated by progressions or transits. Because the nodes are pools of karma associated primarily with the house in which they are located, the related effects are deep and often seem almost inevitable. It is not for nothing that they are called the Dragon's Head and Tail.