Rahu and Ketu: the lunar nodes, explained

Where the Moon’s path crosses the Sun’s

The Moon’s orbit is tilted about 5° to the ecliptic, the Sun’s apparent yearly path. The two orbits cross at two opposite points, the lunar nodes: the ascending node, where the Moon moves north across the ecliptic, is Rahu; the descending node, exactly 180° away, is Ketu.

The nodes are not fixed. The Sun’s pull makes them slide westward — backwards through the zodiac — by about 19.3° a year, completing a circuit in about 18.6 years. That is why Rahu and Ketu are always shown as retrograde.

Why eclipses happen near Rahu and Ketu

Every new Moon passes between Earth and the Sun, but usually a little above or below it, because of the 5° tilt. Only when a new or full Moon falls close to a node do the three bodies line up well enough for an eclipse. The tradition’s image of Rahu and Ketu “swallowing” the Sun and Moon is a mythological reading of this real geometry.

Mean node and true node

The mean node moves smoothly at the average rate. The true node follows the Moon’s actual, perturbed orbit and wobbles around the mean position, at times by more than a degree. The website’s Kundli uses the mean node; the Android app defaults to the true node and lets you choose. Near a sign or nakshatra boundary the choice can move Rahu and Ketu into a different one.

Rahu and Ketu in Jyotish

Jyotish calls them chhaya grahas, shadow planets. Mythology tells of Svarbhanu, whose head and body became Rahu and Ketu. In the Vimshottari dasha Rahu rules 18 years and Ketu 7. These meanings belong to the tradition; the positions above are astronomy.

See the nodes move in the Observatory’s lunar-nodes lesson.