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In short
The mean node is the smoothed, average position of the Moon's ascending node, moving steadily backwards; the true node adds the short periodic swings around it. This app uses the mean node for Rahu everywhere, with Ketu opposite, and computes the true node only to warn when a Kaal Sarp verdict would change on it.
What the term means
The lunar nodes do not slide backwards at a perfectly even pace: the real crossing point wobbles a little either side of an average. The mean node is that average - a smooth position from a formula in time. The true node is the mean node plus the periodic wobble. Software that uses one will place Rahu slightly differently from software that uses the other.
Where the term comes from
The distinction is astronomical, not classical. None of the texts in this site's library - BPHS, Saravali, the Bhrigu Sutras - uses the words mean node or true node, so none says which one its rules were written for. The Bhrigu Sutras give only an average rate, 19 degrees 20 minutes a year backwards (Part I, p. 13), which is a mean-motion figure.
How this app uses it
- meanLunarNode() in astronomy.js computes the ascending node from a polynomial in Julian centuries. computeChart() and currentTransits() both take it, subtract the Lahiri ayanamsa, and call it Rahu; Ketu is that plus 180 degrees.
- trueNodeOffset() adds Meeus's five periodic terms to give the true-minus-mean difference. The code calls it a diagnostic, not a correction: it never moves the chart's Rahu.
- The only consumer of the true node is the Kaal Sarp calculator. It keeps its verdict on the mean node, re-runs the test on the true node, and says so when the two disagree.
How big the gap gets
In a daily run of the engine from 1970 to 2015, the largest gap between true and mean node was 1.88 degrees (23 September 1980). For 19 January 1985, 11:14 IST, the mean node is at Taurus 0.54 and the true node 1.26 degrees further on - still Taurus, so Rahu's sign does not change there.
Pages that use this term
- Kaal Sarp Dosha explained - where the mean-versus-true choice can flip a verdict.
- Why apps disagree on your nakshatra - the node model as one source of disagreement.
- Rahu in Vedic astrology - the node this model places.
- Rahu Mahadasha - a page that names the node model it relies on.
- Kaal Sarp Dosha calculator - reports the true-node check with its verdict.
- Methodology - how the engine computes positions.