The Moon revolves around the earth; the average inclination of its orbit to the ecliptic is 5°- 8'. Owing to this, the Moon assumes North latitude for half year and South latitude for the rest of the year. The plane of the ecliptic and the plane of the Moon's orbit intersect at two points called Nodes. The line joining these points is called the Nodal axis.
The point where the luminary crosses the ecliptic from South to North is called the North Node, and the other is the South Node. The former is named Rahu, the Dragon' head and the latter, Kethu, the Dragon's tail.
Does the Moon really revolve around the earth? In fact they orbit each other around a common centre of gravity about 3000 miles from the centre of the earth! The moon also wobbles like the earth and so the Nodes regress (move backwards) along the ecliptic. Hence the nodes always have a retrograde movement. The nodes take roughly about 18.6 years for one cycle.
Full Moon and New Moon occur when the luminary is aligned in line with the earth and the Sun, but not always at the nodal points, when eclipses take place (Moon assumes 0° latitude).
emergence of Rahu and Kethu is depicted in the Puranas. The king of
the Paradise, Indra was once riding on his elephant Airavata when
he met sage Durvasa known for his anger. The sage offered him the
garland of fortune. Not knowing the merit of the garland Indra put
it on the elephant’s trunk who in turn threw it on the floor.
Enraged by this, the sage cursed that he and his team of demigods lose all their power and fortune.
There is another story too! Vedha Vyasa is considered to be one of the exponents of 'Jyothish', vedic astrology. He has narrated the emergence of the nodes Rahu and Kethu in his 'Shrimad Baghavataham'. Once Sukracharya, the Guru of the asuras, the demons did a great penance and was rewrded the 'Sanjeevini Mantra'. He could by this raise the dead to life.
Knowing this, the demons wanted to defeat the demigods with the help of their guru. Without heeding to the advice of Sukracharya, they started the battle. Duty bound Sukracharya gave life to all the dead asuras and finally, the asuras won the battle.The demons headed by Bali, gained control of the Universe.
The demigods suffered innumerable insults and finally appealed to Lord Vishnu to help them, who advised them to do 'Sagar Manthan' to get the 'Amirdha'. They could regain their lost strength only if they drank Ambrosia, the nectar of immortality, by churning the Ocean of Milk. But it was a great task which could be accomplished
only with the help of the demons. The greedy demons were willing to churn the ocean along with demigods. Their plan was to cheat the other party and have the nectar for themselves. But the divine plan was otherwise!
When the ocean was being churned with the Mandara Mountain as the rod and Vasuki, the snake, as the chord, many invaluable things, gods, and goddesses appeared. At the end Dhanvandiri rose with the pot of nectar.
The demons at once snatched the pot and began to fight among themselves. Lord Vishnu appeared as an enchanting woman (Mohini Avatar). Overcome by her charm, the demons agreed to her plan that the nectar be first offered to the demigods so that the former could have all the rest. The demigods and the demons were seated on opposite rows and the elixir of immortality was being served to the demigods first, by the Mohini.
A wicked idea struck in the mind of one of the demons, Swarbhanu. He assumed the form of a demigod and stealthily sat in the row of the demigods. This was noticed by the Sun who told this to Moon. Moon reported to Lord Vishnu who got so angry that He cut the demon's head. But as the demon had already tasted the nectar he did not lose his life.
After fervent appeals by Rahu and Kethu (the Head and the Tail of the Dragon) they were offered the status of planets with instructions that they go to the ocean of Cancer, learn all the scriptures before assuming office. This was sincerely carried out by the nodes. So the node Rahu always chases the luminaries often trying to swallow them in vain, accompanied by his other part Kethu.