In Hindu mythology, solar eclipses are caused by the decapitated head (Rahu), or lifeless body (Ketu) of the demon Svarbhānu, otherwise known as: “He who darkens the luminaries”.
The story harks back to a time when the asuras (demons) ruled the universe. The suras (demi-gods) had been cursed with ill fortune after Indra (the King of heaven), had insulted a great sage (Durvasa), which allowed the asuras to gain the upper hand in the battle between good and evil.
In desperate need of a way to regain their strength, the suras consulted Vishnu, who advised them to obtain the nectar of immortality. Of course, quests are never simple, and this one came with a catch: it was located on the bottom of the ocean of milk, and they would need to join forces with the asuras to retrieve it.
The suras approached the asuras with the irresistible temptation of immortality and both sides united in a truce with a promise to share in the spoils (both sides also had every intention of reneging on the agreement and taking the nectar for themselves), and so agreed they began their mission. Vasuki, the king of the serpents, wrapped himself around the mountain in the middle of the ocean, and the asuras took hold of his head, while the suras held his tail. They both pulled the giant serpent back and forth, churning the ocean with the mountain as all its secret contents were brought forth, until finally the nectar arose to the surface.
The demons snatched it first and ran away, and the suras once again approached Vishnu and asked for help. In response, Vishnu took the form Mohini, a beautiful woman, and beguiled the asuras into giving her the potion to distribute. The asuras complied to her wishes, stunned into submission by her radiance, and did not dispute when she lined both the suras and asuras into separate lines before her, and began to pour the sweet nectar into the mouths of the suras.
Only one of the asuras, Svarbhānu, realized Mohini’s ruse, and switched lines so that he too would receive the gift of immortality. The sun and moon noticed the darkening that Svarbhānu’s presence brought with him, and called out to Mohini in warning, but it was too late, and she had already tipped the vessel and released the nectar into Svarbhānu’s waiting mouth. Quick as a flash, Lord Vishnu drew his cakra (a special blade which absorbs the user’s chakras) and cut off the head of Svarbhānu before he had a chance to swallow. Svarbhānu’s head went flying into the sky, momentarily blocking the sun and causing an eclipse, while the body fell to the ground. Svarbhānu’s head had already touched the nectar of immortality, and so it lived on as a body without a head known as Rahu, while his body became known as Ketu.
In astrology, Rahu is associated with the north lunar node, while Ketu represents the south lunar node; the places in which the moon crosses with the ecliptic path of the sun (if the crossing of the ecliptic occurs in conjunction with a new moon a lunar eclipse occurs, while a solar eclipse will occur in conjunction with a crossing on a full moon). When the sun and moon cross paths with Rahu or Ketu, they will again darken the luminescence of these astral bodies.
The original version of this story can be found in full in the Bhāgavata Purāṇa (also known as Śrīmad Bhāgavata Mahā Purāṇa and Śrīmad Bhāgavatam), one of the eighteen Purāṇa’s that contain Hinduism’s “Great Histories”. You can get a copy here:
Bhagavata Purana (A Set of Two Volumes)