Rahu: the north node in Jyotish
Rahu is the north lunar node, the point where the Moon's orbit crosses the ecliptic going north. It is treated as a graha in the Jyotish scheme but is not a physical body.
What it is
Rahu's dasha lasts 18 years in the Vimshottari system. Its signs of rulership are not given in the standard table, and the school-dependent details (such as exaltation and aspects) vary between texts.
How it is worked out or read
This site computes Rahu from the mean node formula. The true node differs by up to about 1.5 degrees, and the difference is explained on the mean and true nodes article.
Where it appears on this site
Rahu is shown in the birth chart. The chart checks use Rahu and Ketu for the Kaal Sarpa rule.
Rahu as a point
Rahu is the north node of the Moon's orbit, the point where the Moon crosses the ecliptic moving north. It is treated as a graha in the Jyotish scheme but has no visible disc, so its character is traditionally read through its position and its dasha. Rahu's dasha lasts eighteen years and is one of the longer periods of the cycle. The standard scheme does not give Rahu a sign lordship, and its exaltation and aspects are school-dependent.
How the site computes Rahu
This site uses the mean node, computed from the Meeus formula for the mean longitude of the ascending node. The mean node moves steadily backward at about nineteen degrees a year. The true node differs from it by up to about one and a half degrees because of periodic perturbations. The method page states which is used, and the mean-and-true nodes article explains the difference in full.
Reading Rahu in practice
A reader looks at Rahu's sign and house, and at the lord of the sign it occupies, since that lord links Rahu to the rest of the chart. Rahu is read as an amplifier of the themes of its house and sign, in the classical view, and it is linked with sudden change and unusual paths. The Kaal Sarpa check uses Rahu and Ketu as the two ends of an axis, and the chart checks show whether all seven grahas lie on one side of it.
Rahu and sudden change
Rahu's association with sudden and unusual change is one of its most cited traits. A reader may describe the theme as the pull toward new directions and the possibility of rapid shift, while noting that the classical view also treats it as a source of ambition and reach when well used.
Rahu and the pull of desire
Rahu is sometimes described as the planet of desire and of the pursuit of what is not yet possessed. A reader may describe this as a restless appetite for experience and reach, while noting that its effects depend on the house and sign in which it sits.
Rahu and the axis
Rahu is always paired with Ketu, and many readings consider the two together as an axis across the chart.