Concept · Illustrated
How Rahu and Ketu Are Calculated — The Lunar Nodes Explained
Rahu and Ketu are the most misunderstood points in the chart. They aren't physical planets at all — they're two precise mathematical points in the sky. Here's exactly what they are and how they're calculated.
Every other graha in your chart is a body you could point a telescope at — the Sun, the Moon, Mars through Saturn. Rahu and Ketu are different. They are the chhaya grahas, the "shadow planets": two invisible points in the sky that the ancient seers treated as planets because they shape your life just as powerfully. Understanding what they actually are removes most of the confusion around them.
What Rahu and Ketu really are
The Moon does not orbit the Earth in the same flat plane that the Sun appears to travel (the ecliptic). The Moon's orbit is tilted by about 5°. Because of that tilt, the Moon's path crosses the ecliptic at exactly two points on opposite sides of the Earth. Those two crossing points are the lunar nodes — and that is all Rahu and Ketu are.
- Rahu is the ascending node (Sanskrit Rahu — the north node): the point where the Moon crosses the ecliptic moving northward, from below the plane to above it.
- Ketu is the descending node (the south node): the point where the Moon crosses moving southward.
Because they are two ends of the same line through the Earth — the line of nodes — Rahu and Ketu are always exactly 180° apart. This is why, in Vedic software like the chart engine on this site, Ketu's position is simply computed as:
Ketu's longitude = Rahu's longitude + 180°
You never calculate Ketu separately. Find Rahu, add half a circle, and you have Ketu directly opposite.
How the position is actually calculated
To place Rahu in a chart, an astronomer (or an ephemeris engine) computes the longitude of the ascending node of the Moon's orbit at the exact date, time and place of birth. Two refinements then make it specifically Vedic:
- Mean node vs true node. The Moon's orbit wobbles, so the node is constantly shifting. The mean node is the smoothed, averaged position; the true node is the exact instantaneous position, which oscillates around the mean. Classical Jyotish — and this site's engine — uses the mean node, the traditional standard. (The two rarely differ by more than about 1.5°.)
- Sidereal zodiac (ayanamsa). Vedic astrology measures positions against the fixed stars, not the seasons. So the raw nodal longitude is shifted back by the Lahiri ayanamsa (the Indian government standard, ~24°) to give the sidereal position used in your kundli. The same correction is applied to every planet — see how to read your birth chart for the bigger picture.
A third quirk: because the nodes drift backwards through the zodiac, Rahu and Ketu are always retrograde. They never move direct.
Why they cause eclipses (and the mythology)
The nodes matter astronomically because eclipses can only happen near them. A solar or lunar eclipse occurs when the Sun and Moon line up at or near a node — that is, when all three bodies fall on the same plane. This is the kernel of truth inside the famous myth: Rahu, a demon who stole a sip of the nectar of immortality, was beheaded by Vishnu. His severed head (Rahu) and body (Ketu) live on, and in revenge they periodically "swallow" the Sun and Moon — which is precisely an eclipse, happening at the nodes.
This is why the nodes carry such intense, fated, larger-than-life symbolism in astrology. They are literally the places where the lights of the chart get devoured.
What they mean in your chart
Because they are points of cosmic tension rather than ordinary planets, the nodes work as a karmic axis:
- Rahu is the direction of insatiable worldly desire — where you over-reach, obsess, and are pulled to grow in this lifetime. Read the full Rahu guide →
- Ketu is the point of detachment and past-life mastery — where you already have depth, so you let go and turn inward toward moksha. Read the full Ketu guide →
They always sit in opposite houses, defining the axis your soul is working along: what to engage (Rahu) and what to release (Ketu).
FAQ
Are Rahu and Ketu real planets?
No. They are not physical bodies. They are the two mathematical points where the Moon's tilted orbit crosses the ecliptic — the lunar nodes. Vedic astrology treats them as "shadow planets" (chhaya grahas) because of their strong influence, not because they are objects in space.
Why are Rahu and Ketu always opposite each other?
Because they are the two ends of a single straight line — the line of nodes — passing through the Earth. Opposite ends of a line through a centre are always 180° apart, so Rahu and Ketu are always exactly six signs apart.
What is the difference between mean node and true node?
The mean node is the averaged, smoothed position of the lunar node; the true node is its exact instantaneous position, which oscillates slightly around the mean. Traditional Vedic astrology uses the mean node, and so does this site's chart engine. They usually differ by less than 1.5°.
Why are Rahu and Ketu always retrograde?
The line of nodes slowly drifts backwards (westward) through the zodiac, completing a full cycle in about 18.6 years. Because their motion is always in the reverse direction of the planets, Rahu and Ketu are considered permanently retrograde.
How do Rahu and Ketu relate to eclipses?
Eclipses can only occur when the Sun and Moon align near a node, so that the Earth, Sun and Moon fall on the same plane. In mythology, Rahu and Ketu "swallow" the Sun or Moon at these moments — which is exactly what an eclipse is.