Rahu & Ketu

According to the Sanskrit epic poem, the Mahabharata, the Hindu gods decided to mix up a batch of soma, the elixir of immortality. The gods were to drink the elixir to become immortal. The gods needed help from the demons to stir up the oceans to produce the elixir. Out of the churning oceans the Sun, Moon, many goddess, and magic things were produced along with the soma. Vishnu took charge of distributing the freshly made soma to the gods but while it was being passed out the demons started battling with the gods for a taste of the elixir and in the confusion one of the demons, Rahu, disguised himself as a god and drank some of the elixir. The Sun and Moon spotted the imposter, Rahu, and told Vishnu. Just as Rahu was swallowing the soma, Vishnu sliced off his head with a sword. Since the soma had passed into the throat the head had already become immortal and remained alive. Because the Sun and the Moon were responsible for reporting the misdeeds of Rahu, Rahu, the head, chases the Sun and Moon across the sky and tries to eat them. Occasionally he catches and swallows one of them, causing an eclipse. But the victim quickly falls out of Rahu's throat and the eclipse ends. (Some believe that banging on drums or utensils during the eclipse helps to scare Rahu into releasing the Sun or Moon.) Apparently Ketu also chases the Sun and Moon and can also cause eclipses. (How the headless body swallows either the Sun or Moon is not clear to the author.)

Hindu astronomers, from the time of Gupta (about 300 a.d.), understood that eclipses were indeed caused by the shadows of the Earth or Moon, depending, and that the story of Rahu and Ketu was just a story. However they also understood that an eclipse can only occur only when the Sun and Moon are close to the point in their respective orbits where the path of the Sun crosses the path of the Moon. The Sun moves against the background of stars around the entire sky in the course of a year. This path is called the ecliptic. The Moon moves around the entire sky in the course of about 27 days, and its path is such that it crosses the ecliptic in two places. The place where the path of the Moon crosses the ecliptic moving northward is called the ascending node and the place where the path of the Moon crosses the ecliptic moving south is called the descending node. Thus the Moon crosses the ecliptic at the ascending node, roughly 14 days later crosses the ecliptic again at the descending node, and roughly 14 days later it is back to the ascending node crossing.

This figure shows the path of the Sun against the background of stars (yellow), the path

of the Moon above and below the path of the Sun (white),the ascending node, the

descending node, and the figures of the constellations of the zodiac (blue). (The chart was

made with Voyager II software and annotated with Adobe Photoshop.)

If it happens that the Sun is near one node and the Moon is near the same node, at the same time, it is likely there will be an eclipse of the Sun. If the Sun is near one node and the Moon is near the other node, at the same time, it is likely there will be an eclipse of the Moon. Things are somewhat complicated by the fact that the nodes move very slowly from one month to the next making it more difficult to predict eclipses. However Hindu astronomers discovered that if they kept track of the location in the sky of the nodes, then they only need to check the proximity of the Sun or Moon to the nodes in order to be able to predict an eclipse. Since the nodes are imaginary points they are referred to as "shadow planets" and are named to honor the mythical characters; the ascending node is called Rahu and the descending node is called Ketu. Adding Rahu and Ketu to the seven visible planets, Sun, Moon, Mercury, Venus, Mars, Jupiter, and Saturn, Hindu astronomers had a total of nine planets.

This page prepared by C. Hartley, Director of the Ernest B. Wright Observatory at Hartwick College in the City of Oneonta, NY. Contact : hartleyc@hartwick.edu