Ketu: the south node in Jyotish
Ketu is the south lunar node, always exactly opposite Rahu, and is treated as a graha in the Jyotish scheme.
What it is
Ketu's dasha lasts 7 years and it opens the Vimshottari cycle. Like Rahu, it has no sign lordship in the standard table, and its school-dependent details vary.
How it is worked out or read
Ketu is always 180 degrees from Rahu in this site's computation, so the two are always in opposite signs.
Where it appears on this site
Ketu is shown in the birth chart. The dasha tool begins its sequence with Ketu.
Ketu as the opposite point
Ketu is the south node of the Moon's orbit and is always exactly opposite Rahu. Where Rahu is read as an amplifier of its house, Ketu is read as a detachment from the themes of its house, in the classical view, and as a point of release. Its dasha is seven years, and it opens the Vimshottari sequence, so the first dasha at birth may be Ketu's when the Moon is in Ashwini, Magha or Mula.
Ketu in the computed chart
Because Ketu is computed as Rahu plus 180 degrees, the two are always in opposite signs and opposite houses. A reader who learns Rahu's sign can find Ketu's sign by counting six signs forward. The houses are opposite too, so Rahu in the fourth house means Ketu in the tenth. The birth chart shows both, and the chart checks use both for the Kaal Sarpa rule.
Reading Ketu with the same care
Like Rahu, Ketu has no standard sign lordship, and its details vary between schools. A reader should not take an unsourced description of Ketu as settled. The safest reading uses Ketu's sign and house, the lord of the house it occupies, and the planets that aspect or join it, and it states any school-specific claim as such.
Key facts
Ketu is the descending, or south, lunar node, always exactly opposite Rahu. Its Vimshottari period is seven years, and it rules the nakshatras Ashwini, Magha and Mula, the first nakshatras of the three rounds of nine. It has no day of its own in the weekday scheme. In the Puranic story, Ketu is the headless body of the asura whose head became Rahu.
Motion
Like Rahu, Ketu moves backwards through the zodiac at about three minutes of arc a day on average, taking about eighteen months to cross a sign. This site uses the mean node, so Ketu's position is always exactly 180 degrees from Rahu's. Eclipses happen when the Sun and Moon are near Rahu or Ketu.
Reading Ketu
Ketu is traditionally associated with detachment, spiritual insight, sudden events and release, and is called the karaka of moksha. It is often described as acting like Mars. Its house is read as an area where the person may feel unattached or experienced, and the Rahu-Ketu axis together is often read as a balance between worldly pursuit and letting go.