Rahu and Ketu -- How a Demon Lost His Body and Became the Enemy of Sun and Moon

Every few months, somewhere on earth, the sun goes dark in the middle of the day or the moon turns blood-red at night. Modern astronomy explains eclipses with orbital mechanics -- the moon's shadow crossing the earth (solar eclipse) or the earth's shadow crossing the moon (lunar eclipse). Hindu mythology explains them with a grudge.

The story begins at the Samudra Manthan -- the Churning of the Ocean of Milk -- one of the most important episodes in Hindu cosmology. Devas and Asuras cooperated to churn the cosmic ocean using Mount Mandara as the churning rod and the serpent Vasuki as the rope. The goal: to extract Amrit, the nectar of immortality.

When the Amrit finally emerged, carried in a golden pot by Dhanvantari, the Asuras grabbed it. Chaos erupted. To restore order, Vishnu assumed his Mohini avatar -- the enchanting female form -- and offered to distribute the Amrit fairly. The Devas sat in one line, the Asuras in another. Mohini began serving the Devas first.

One Asura -- Svarbhanu -- was not fooled. He saw through the strategy. While Mohini served the Devas, Svarbhanu quietly slipped out of the Asura line, disguised himself as a Deva, and sat between Surya (the Sun) and Chandra (the Moon) in the Deva line. When Mohini reached him, she poured the Amrit into his mouth.

But Surya and Chandra recognised the imposter. They alerted Vishnu. In that instant, before the Amrit could pass below Svarbhanu's throat, Vishnu hurled his Sudarshana Chakra and severed the demon's head.

The Amrit had already touched Svarbhanu's mouth and throat. His head and body both became immortal -- but separated forever. The severed head, still alive and raging, became Rahu. The headless torso, writhing in darkness, became Ketu. And their eternal vendetta was set: Rahu would forever chase and swallow the Sun, Ketu would forever chase and swallow the Moon -- the two beings who had betrayed them. Each time they catch their target, an eclipse occurs. But because Rahu has no body and Ketu has no head, the swallowed celestial body passes right through -- and the eclipse ends.

This is the Puranic explanation for eclipses. It appears in the Vishnu Purana, Bhagavata Purana (Skandha 8), Matsya Purana, and Mahabharata (Adi Parva).