The lunar nodes are the two points where the Moon's orbit, tilted about five degrees to the ecliptic, crosses the plane of the ecliptic. The North Node, also called the ascending node, is where the Moon crosses going north, and it is where eclipses occur when a new or full Moon falls near it, which is the reason the nodes were tracked so carefully by ancient astronomers. They move backwards through the zodiac, completing a circuit in about eighteen and a half years, roughly a sign and a half per year. In Indian astrology the North Node is Rahu, treated as a full planetary agent, a chhaya graha or shadow planet, and in the myth it is the severed head of the demon who drank the nectar of immortality and now swallows the Sun and Moon in eclipse. Western tradition knew it as the Dragon's Head, caput draconis, and read it as increasing whatever it touched.
The Saros cycle, the roughly eighteen-year, eleven-day period after which the Sun, Moon and nodes return to nearly the same relative configuration and a similar eclipse recurs, was tracked by Babylonian astronomers centuries before Hellenistic astrology adopted the nodes as chart points, and it remains the basis on which modern astronomers still group and number eclipses into numbered Saros series today, a rare case of a genuinely ancient predictive astronomical technique that continues in unmodified practical use in modern professional astronomy.