Rahu and Ketu are called shadowy planets by our ancients. They are not substantial heavenly bodies with an observable body shape or mass like the Sun, Moon and other planets. Hindu mythology has in its characteristic way symbolised these two parts of a single body. According to a Puranic story after churning of the ocean (Samudra Manthan) came out a divine elephant, a horse, a physician, the Goddess of wealth and Amrita or immortality. About the distribution of Amrita there was a serious dispute amongst the Devas and Asuras. The Supreme Lord (Lord Vishnu) assuming the shape of an alluring Mohini, took charge of distributing the Amrita with the consent of both Devas and Asuras. She so distributed the Amrita that all of it was consumed by Devas but one among the Asuras had stealthily joined the ranks of Devas and got a share of Amrit but was discovered and exposed by the Sun and the Moon. The Supreme Lord cut the Asura in two pieces but as he had taken Amrita both the severed parts remained alive though separated. The head got the name of ‘Rahu’ and the body and tail ‘Ketu’. This made Rahu and Ketu eternal enemies of the Sun and the Moon.

Periodically they succeed in swallowing the Sun and the Moon (This is during eclipses) but they escape as they are immortal. The fact that has since been universally accepted is that Rahu Ketu are celestial points on the Zodiac with regulated movement and orbit and they have a distinct and predictable influence in horoscopes. The motion of these points which is always retrograde, can be calculated as accurately as the position of the Sun or the Moon, through owing to their being without substance, this cannot be verified by visual observation. Westerners have named Rahu and Ketu as Dragon’s Head and Dragon’s Tail respectively. Caput and Cauda are the other names given to them by Westerners. To understand how these imaginary points are accurately located and their movements are calculated we have to consider the apparent paths in the heavens of the two planets the Sun and the Moon, that is, the paths along which the Sun and the Moon appear to an observer on earth, to rotate. The Sun appears to move along an ellipse of which Earth is a focus. The plane of this ellipse is inclined to the terrestrial Equator at an angle of 23 degrees. The Moon, as the satellite of the Earth moves round it and its path is also elliptical with the Earth as a focus, and plane of this orbit is inclined to the Equator at an angle of 28 degrees, so that the planes of the orbits of the Sun and the Moon appear to the observer on Earth to be inclined to each other at an angle of 5 degrees. The two points at which the orbit of the Moon cuts the orbit of the Sun are called nodes, and they are exactly 180 degrees apart. The point of intersection formed when the Moon’s motion is from South to North, is said to be ascending node and named Rahu, and the point of intersection formed when the Moon’s motion is from north to south is called the descending node or Ketu. It has been observed that the nodes have a retrograde movement in the Zodiac at the rate of 19 degrees 20 minutes per year. Rahu and Ketu are said to stay in a sign for about 18 months.

Rahu and Ketu indicate one born in a low caste; one who may have intrigue with girls; is one whose thoughts and actions will be evil. One will resort to outcastes and low born as well as in bred people. There is quite a controversy amongst the ancients about the signs owned by Rahu and Ketu and signs in which they are exalted and debilitated. For example, according to Uttara Kalamrita an ancient classic on Hindu Predictive Astrology Rahu is exalted in Taurus and debilitated in Scorpio, vice versa of Ketu. This work also assigns the sign Aquarius as owned by Rahu and Scorpio as owned by Ketu. Mooltrikona of Rahu has been fixed as Gemini and Mooltrikona of Ketu has been given as Virgo.

Sanketnidhi another recognised work allots Gemini as the exaltation sign of Rahu, Virgo as his own sign and Sagittarius as the exaltation sign of Ketu and Pisces as its own Sign. It is also believed that Rahu acts like Saturn and Ketu acts like Mars. The Gem of Rahu is “Gomed” and that of Ketu Cat’s Eye.