Introduction
Rahu is the head of the Asura Svarbhanu, whose body is Ketu. He is one of the nine celestial bodies (Navagraha) of Hindu astrology, and is the ascending lunar node, the point where the moon's path crosses the sun's path. In mythology, he is the cause of solar and lunar eclipses, when he swallows the sun and moon.
The story
During the churning of the ocean of milk (Samudra Manthan), the gods and demons together obtained the nectar of immortality (amrita). Vishnu, as the enchantress Mohini, tricked the demons and began to distribute the nectar to the gods. The Asura Svarbhanu, son of Vipracitti and Simhika, disguised himself as a god and sat between the sun and the moon. They recognised him and told Vishnu, who cut off his head with his discus, but the nectar had already reached his throat. His head stayed immortal, and became Rahu, and his body became Ketu.
Eclipses
Rahu is said to take revenge on the sun and moon by swallowing them at the time of eclipses. Since he has no body, they pass through his severed neck and appear again. This is why, during eclipses, many Hindus fast and perform rituals, bathe after the eclipse, and give donations.
In astrology
In Jyotisha, Rahu is a "shadow planet" (chhaya graha) that has no physical body but a strong influence. He is associated with ambition, obsession, illusion, foreign things and sudden change. Rahu is said to rule the period called Rahu Kalam, a period of about ninety minutes each day, which is regarded as inauspicious for beginning new work. Kala Sarpa Dosha is a configuration in a birth chart in which all planets are between Rahu and Ketu.
Iconography
Rahu is shown as a head without a body, or as a figure with a dragon or a snake tail, riding a lion or chariot pulled by eight black horses.
Astronomy
Rahu and Ketu correspond to the lunar nodes, the points in the sky where eclipses can happen, which Indian astronomers understood from ancient times.
Worship
The Srikalahasti Temple in Andhra Pradesh is famous for Rahu-Ketu rituals. The Thirunageswaram temple in Tamil Nadu is associated with Rahu.
In other cultures
In Buddhism, Rahu (Rahu Asurinda) is the chief of the Asuras and is said to seize the sun and moon, and and one traditional account connects the name of the Buddha's son Rahula with the day of a lunar eclipse on which he was born. In Thailand and Burma, Rahu is shown as a face swallowing the moon, and is honoured in temples.