Ketu, the shadow planet in Vedic Astrology, is the south lunar node in Astronomy. Lunar nodes are points that mark the intersection of the Moon’s orbit around the Earth with the ecliptic (the apparent path of the Sun in the sky). Thus, the nodes are merely the interception of these two orbit lines and the south node is the point where the orbit of the moon crosses to the south (or descending) of the ecliptic.
Position of Ketu is significant in both astronomy and Vedic astrology. When on a New Moon day, the Sun and Moon are in conjunction and Ketu is also in conjunction with both, a solar eclipse occurs. When the Sun and Moon are in opposition on a Full Moon day, and one of them is in conjunction with Ketu, a lunar eclipse occurs. In Vedic Astrology, ketu represents the headless part of the body of a demon (Rahu is demon head). Ketu is malefic but less compared to Rahu. It stands for spirituality, liberation from karmic cycle and non-attachment to earthly desires and ambitions. It also stands for wisdom, psychic power, healing ability. It is against materialism unlike Rahu which is for materialism. When malefic or in conjunction with malefics, it is destructive like Mars. It stays in any zodiac sign (Rasi) for a period of about one and half a year and completes one revolution around the zodiac in eighteen and half years. Indian Astrological treatises like Brihat Parashari Hora outline predictions resulting from Ketu