Overview
Ketu (Sanskrit: केतु, IAST: Ketú) is the descending or south lunar node in Vedic (Hindu) astrology. Together with Rāhu, the ascending or north lunar node, Ketu is regarded as one of the navagraha (the nine celestial influences), although the two are not physical planets. Astronomically, Rāhu and Ketu mark the points where the apparent paths of Surya (the Sun) and Chandra (the Moon) intersect on the celestial sphere.
In Hindu tradition, Rāhu and Ketu are personified as deities and are described as the two halves of the immortal asura Svarbhanu, who was beheaded by the god Vishnu. According to this narrative, eclipses occur when the Sun and the Moon arrive at one of these nodal points and are said to be swallowed by Rāhu or Ketu. Ketu is traditionally held to be responsible for causing the lunar eclipse.
In Vedic astrology, Rāhu and Ketu are described as having an orbital cycle of 18.6 years and as always being positioned 180 degrees from each other, both orbitally and within birth charts. This corresponds to the 18.6-year precessional cycle of the lunar ascending and descending nodes on the Earth's ecliptic plane. Ketu is associated with the zodiac sign Virgo, which it is said to rule along with Budha (Mercury, the traditional ruling planet of the sign).
References
Adapted from the English Wikipedia article on Ketu (mythology).