Raghu and Kethu are two entities of the same demon in Vedic Astrology. The story of swarbhanu a dragon goes like this. Its desire was to bring out the gems lying hidden in the ocean. To bring the gem stones hidden in the ocean is not important but what is meant here is the secrets were unknown so far. In spiritual literature water is the base of all manifestation and is used as a purifying agent in all religions of the world. The gods and demons were represented as good and evil forces in Hindu mythology. The evil forces wanted to churn the ocean. It was decided to use all the available forces of the gods and demons (Suras and Asuras) As the result would be more important they decided to work together and share all that came out. So Meru Mountain was selected as the stick for churning. A serpent named as Aasthi sesha was used as the cord for the stick. Many gems came out of the sea one by one at the time of churning. At last Amirtha the nectar that makes one immortal to one who drinks came out from the sea.
The gods dislike to share this nectar with the demons, let them become immortal and evil perpetuate. The demons were already afraid of the intentions of Gods and approached to Lord Vishnu appeared before the quarrelling parties in the form of a beautiful lady. The damsel assured justice to both parties and asked them to stand in two rows. Swarbhanu one of the demons afraid of the justice passed into the row of Gods and got his share of the nectar. It was only after drinking the nectar that he was detected by Sun and Moon who complained about it to Lord Vishnu who got angry and cut off the head of the demon. The two portions head and the trunk have been going round in the Zodiac. As Sun and Moon are both their enemies they try to eat them which are the cause of solar and lunar eclipses. Swarbhanu before drinking the nectar had imposed by Tapas and got a boon from Lord Brahma that he would be worshipped along with all the planets Sun and Moon. On account of this boon of Lord Brahma that Lord Vishnu (the damsel) ignored and gave him the nectar. The head portion of the demon is known as Rahu and the trunk as Kethu. Churning of the ocean for taking out the gems was to have the secrets of life. The mount meru is nothing but our spinal cord. The nectar that appeared last of all was the secrets of life and death. The serpent used as a rope to bind the mountain is the wise man who guided the operation. In short the churning of the sea is awakening and activating the serpent power. The person who detected the demon was moon who represents an illuminated mind. An illuminated mind can distinguish between good and bad Head is the governing portion of the entire body and knowledge is deposited in the lower portion of the body. The body cannot be utilized by a headless man. Chopping off the head means a separation of the governing part of the body from real truth.
The head comprising the mind and power to govern the body is termed as Rahu. As Rahu has gained the secrets of life and death he becomes invisible. He will try to get what he wants. In other words speaking astrologically Rahu will get whatever it stands for by hook or by crook. It is compared with Saturn. Rahu in eclipses is active in dark fortnight and Kethu in bright fortnight. Raghu and Kethu have no house of their own. They are guided by the lord of the house they occupy. They are also influenced by the planets. Rahu is the head of the dragon. Head contains the brain and controls the functioning of the body. Kethu rules of the body. Kethu has the power of transforming the events into wisdom the wisdom derived from events is stored by Kethu. Kethu stores the past events in its own way and delivers the results through the agency of its sign and the planets connected with it.