Rahu & Ketu: The Nodal Axis

One Axis, Two Ends

What the nodes are, why they are always opposite, and how the pair moves.

Two points, one line

The Grahas chapter introduced Rahu and Ketu as chhaya grahas, shadow planets. Here is the geometry behind that.

The Moon's orbit is tilted about 5° to the Sun's apparent path, the ecliptic. The two paths cross at two points. Rahu is the crossing where the Moon heads north, and Ketu is the crossing where it heads south.

Those two points are the two ends of one line through the Earth, which is why they are always exactly 180° apart. So this chapter never reads Rahu without Ketu. When one moves, the other moves. When one sits in your 10th house, the other sits in your 4th.

Why the nodes mean eclipses

An eclipse needs the Sun, Moon and Earth lined up in three dimensions, not just on a flat chart. At most new Moons the Moon passes a little above or below the Sun, and nothing happens.

An eclipse happens only when a new or full Moon falls near the nodal axis: a solar eclipse at a new Moon, a lunar eclipse at a full Moon. The Sun crosses the axis about every six months, so eclipses come in seasons.

The old story of a demon swallowing the Sun and Moon records exactly where eclipses happen. That is why the nodes carry the eclipse mythology, and why the Sun and Moon meeting the nodes gets its own level later in this chapter.

Mean node or true node?

There are two ways to compute the nodes.

The mean node is the smoothed average position. It glides steadily. The true node is the actual crossing at that instant, and it wobbles back and forth around the mean.

This app uses the mean node, and places Ketu exactly 180° from Rahu. How much does the choice matter? When we ran the engine day by day from 2000 to 2040, the true node was never more than about 2° from the mean. It sat in a different sign about 3% of the time.

So if another program puts Rahu a degree or two away from this app, check its node setting before assuming either one is wrong.

Always backwards, about 18 months a sign

The mean node moves backwards through the zodiac (retrograde) at about 3 arcminutes a day. It never stops and never turns direct. That is why the app always shows Rahu and Ketu as retrograde. (The true node's wobble does briefly carry it forwards, about a quarter of the time in that same engine run.)

The pace is very regular:

• About 566 days per sign, roughly 18.6 months
• About 18.6 years for a full lap of all twelve signs

Backwards means Rahu leaves Aries for Pisces, not Taurus. And both nodes change sign on the same day, into opposite signs.

Following the pair

Rahu has just entered Aquarius, at 29°59′ of the sign, because it arrives from the Pisces side moving backwards. On the same day Ketu enters Leo.

• About 18.6 months later: Rahu enters Capricorn and Ketu enters Cancer.
• After six signs, about 9.3 years: Rahu has reached Leo and Ketu has reached Aquarius. The ends of the axis have swapped places.
• After twelve signs, about 18.6 years: Rahu is back in Aquarius and Ketu is back in Leo.

Every step moves both ends at once, so the pair always marks out one axis across the wheel.

Remember it

One line, two ends, always opposite, always backwards. 18.6 months a sign, 18.6 years a lap. The digits repeat because there are twelve signs and twelve months. That is arithmetic, not astrology, but it makes a good memory hook.

Terms introduced here

One axis, two ends

Rahu and Ketu are the two points where the Moon's orbit crosses the Sun's path. They are always exactly 180° apart and are read as one unit.

The eclipse connection

Eclipses happen only when a new or full Moon falls near the nodal axis. That is where the nodes' eclipse mythology comes from.

How the axis moves

The mean node moves steadily backwards: about 18.6 months per sign and about 18.6 years around the zodiac. This app uses the mean node.