Since the plane of the Moon's orbit is tilted at an angle to the earth's orbit, the planes intersect at a line, called the line of nodes. The node where the two intersect as the Moon is crossing the earth's orbit travelling northward is called the ascending, or north, node. The opposite node is the descending, or south, node.

Any planet conjunct (either) point at birth is greatly influenced by this phenomena...an eclipse, for instance, occurs when either Sun or Moon is conjunct a node. Without a doubt they do have a very strong and specific influence on the planets they conjoin and, because of this, they're treated as planets in Vedic astrology, however, as they have no physical body, they're known as the shadowy planets and their influence is confined to the house they occupy, the planet(s) they conjoin and their dispositor (the planet who's house they occupy).

The north node (Rahu) is compulsive and extroverted by nature and will draw us compulsively towards the significations of the planet he conjoins and the affairs of the house he occupies. Rahu draws us to the things of the world and in the 10th house, for instance, can indicate great fame. The south node (Ketu) is diametrically opposite in nature; shy, introverted and restrictive. His influence is to detach us from the significations of the planet he conjoins and the affairs of the house he occupies. Because of this quality of non-attachment Ketu is known as Moksha Karaka...moksha means enlightenment and karaka means significator, therefore, Ketu is the significator of enlightenment.

Mythology: According to Hindu mythology the nodes are the two halves of a rakshasa (demon) who concealed himself in the midst of the planetary beings when they drank the amrita of immortality distributed by Lord Vishnu. He was discovered by the luminaries (Sun & Moon) only after he'd drank of the nectar, thereby becoming immortal (along with the other divine planets) with the ability to manifest each time a being is born. Upon his discovery Lord Vishnu immediately severed the demons head with his chakra. The head became Rahu and the bottom half became Ketu (the headless trunk).

A rakshasa (in general) is a type of demon who has the power to change into any form including animals, dragons, monsters or, in the case of a female (rakshasi), into a beautiful woman. In this way the rakshasa, who was to become Rahu and Ketu, concealed himself amidst the planetary beings.

Frequency of Eclipses (sourced from flycapers.com): As the Moon orbits the Earth, the Sun, Earth and Moon approximately line up about every 29.5 days. At this time the Moon is either at New Phase (when the Moon is between the Earth and Sun) or Full Phase (when the Earth is between the Moon and Sun). However, the Moon usually does not pass in front of the Sun (causing a solar eclipse), nor does the Moon pass into the Earth's shadow (causing a lunar eclipse) since the Moon's orbit is inclined about 5 degree to the plane of the Earth's orbit around the Sun.

Solar and lunar eclipses can only occur when the Moon is near (±18¡) one of the two points where the Moon's and Earth's orbits cross (called nodes). This allows the Moon to pass at least partially in front of the Sun (a solar eclipse), or at least partially move into the Earth's shadow (a lunar eclipse). That is, for eclipses to occur the Moon must be at New or Full Phase and the line of nodes must point toward Sun (as in points 1 or 3 below). Otherwise the Moon will not lie in the plane of the Earth's orbit and eclipses will not be possible (such as point 2).

Lunar Nodes and Eclipses: The Moon's orbit is inclined about 5¡ to the plane of the Earth's orbit around the Sun.

Every six months the nodes line up with the Sun's direction and solar eclipses become possible at the time of New Moon. Likewise lunar eclipses become possible at the time of Full Moon. These two intervals of time when solar and lunar eclipses are possible are called eclipse seasons. Because the Moon's orbit slowly rotates in space, the nodes regress slowly with a period of approximately 18.6 causing the two eclipse seasons to occur about 19 days earlier each year.

Wendy Vasicek, PGA