Rahu and Ketu correspond to the points where the moon crosses the ecliptic plane (known as the ascending and descending nodes of the moon). Classically known in Indian and Western astrology as the head and tail of the dragon, these planets are represented as a serpent-bodied demon beheaded by the Sudarshan Chakra of Vishnu after attempting to swallow the sun. They are primarily used to calculate the dates of eclipses. They are described as shadow planets because they are not visible in the night sky. Rahu has an orbital cycle of 18 years, Ketu has an orbital cycle of 7 years and they are always retrograde in motion and 180 degrees from each other.