Astronomy
Rahu and Ketu have been considered as Grahas in Vedic astrology. They have no physical identity but only mathematically calculated sensitive points having immense influence on the natives of earth.
Moon in its orbit, on a northerly course from South to North, crosses the ecliptic (the apparent path of Sun). This point of intersection is known as Rahu or Dragons head. 180 degrees away from this point, Moon on its southerly course crosses the ecliptic. This point is called the dragon’s tail or Ketu.
Rahu and Ketu are unique to our system or astrology. Why so much stress on the point of intersection of the ecliptic is given has to be understood. While sun is the body and Moon is the mind the intersection is bound to give immense effect.
Rahu and Ketu are not stationary in space but have a mean motion of about 19 degrees and 30 minutes in a year. It means it takes about 18 years and 6 months to make a revolution around the Earth. This motion is retrograde. There is a concept of Mean Rahu and True Rahu. In Hindu astrology we take the true position of Rahu and Ketu.