The 1.13 Degrees That Can Move Diwali by a Month
Published 11 September 2026 · Last updated 11 September 2026
Diwali is the new moon that falls with the Sun in sidereal Libra. That single rule hides a setting. Where sidereal Libra begins depends on the ayanamsa, and two widely used ayanamsas differ by just over one degree. Across 36 years they agree on 34 Diwali dates. In 2001 and 2020 they disagree, and when they disagree the festival moves by a whole lunar month.
This page was written because our own engine got 2020 wrong. Running the rule naively against the ayanamsa this site uses returns 16 October 2020 for Diwali. The observed festival, and the exchange muhurat session, were on 14 November. The gap is not an error in the rule. It is the rule meeting a boundary, and it is worth publishing because almost no date page anywhere states which setting produced its answer.
The mechanism, in one calculation
A sidereal longitude is the tropical longitude minus the ayanamsa. At the new moon of 15 November 2020, 06:00 UT, the Sun stood at tropical longitude 233.3288 degrees. Subtract each system:
| System | Ayanamsa | Sidereal Sun | Sign |
|---|---|---|---|
| PVRN Rao True Pushya Paksha | 23.0156 | 210.3132 | Scorpio, by 0.31 degrees |
| Lahiri | 24.1487 | 209.1801 | Libra, by 0.82 degrees |
Sidereal Libra ends at 210 degrees. The two systems sit 1.1331 degrees apart, and that November new moon falls inside the gap. Under Lahiri the Sun is still in Libra, so this is the Kartika new moon and Diwali follows it. Under True Pushya Paksha the Sun has crossed into Scorpio, so the previous new moon of 16 October becomes the Kartika one. One degree of setting, one month of festival.
The Sun moves about one degree a day, so the two systems are roughly 27 hours apart on every sign boundary. For almost every year that margin is irrelevant. For a new moon that lands inside it, the consequence is not 27 hours. It is 29.5 days, because the boundary decides which lunation gets the name.
How often it happens
Every new moon between August and December was computed for 36 years, 2000 to 2035, under both systems. A sample of the results, including both disagreements:
| Year | New moon chosen, True Pushya Paksha | New moon chosen, Lahiri | Result |
|---|---|---|---|
| 2000 | 2000-10-27 | 2000-10-27 | Agree |
| 2001 | 2001-10-16 | 2001-11-15 | Disagree by one lunar month |
| 2002 | 2002-11-04 | 2002-11-04 | Agree |
| 2009 | 2009-10-18 | 2009-10-18 | Agree |
| 2012 | 2012-11-13 | 2012-11-13 | Agree |
| 2020 | 2020-10-16 | 2020-11-15 | Disagree by one lunar month |
| 2024 | 2024-11-01 | 2024-11-01 | Agree |
| 2025 | 2025-10-21 | 2025-10-21 | Agree |
| 2026 | 2026-11-09 | 2026-11-09 | Agree |
| 2031 | 2031-11-14 | 2031-11-14 | Agree |
Two disagreements in 36 years, 2001 and 2020. In both, the panchang convention used across India follows Lahiri, and the observed festival matched it. Several other years sit close to the edge: 2012 at 209.07 degrees and 2031 at 209.16 under True Pushya Paksha are inside one degree of the boundary, which means a third system with a slightly different value could split them too.
What this means for date pages, including ours
A festival date page that does not state its ayanamsa is giving an answer without giving the setting that produced it. For 34 years in 36 that omission costs nothing. For the other two it produces a page that is wrong by a month while looking exactly as confident as the others.
Our own position: this site computes under True Pushya Paksha and says so on every page. For festival observance we follow the convention actually in use, which is the Lahiri derived panchang, and where the two diverge we state both. That is the honest arrangement. A research engine can hold one ayanamsa and still acknowledge that a festival is a social fact settled by observance.
We checked our own published 2026 dates against this. The November 2026 new moon sits at 203.83 degrees under True Pushya Paksha and 202.70 under Lahiri, both comfortably inside Libra, so the Dhanteras and Diwali page is safe under either system.
The wider point about stated settings
We checked the five largest kundli matching sites in India on 16 August 2026 and not one of them states which ayanamsa it uses. The same number that can move a festival by a month moves every planet in every chart those sites compute, sometimes across a sign boundary. A reader has no way to reproduce a result from a system that will not name its own constant.
Method
New moon instants were located by scanning the Moon and Sun elongation hourly and recording each pass through zero. At each instant the Sun's sidereal longitude was computed twice, once under the PVRN Rao True Pushya Paksha ayanamsa and once under Lahiri. The Kartika new moon is the one at which the Sun occupies sidereal Libra, 180 to 210 degrees, and where two qualify the later is taken. Positions from the Swiss Ephemeris.
Questions
Why do different calendars show different Diwali dates?
Usually they do not. Two widely used ayanamsas agree on 34 of 36 Diwali dates between 2000 and 2035. They disagree in 2001 and 2020, and because the disagreement decides which new moon begins Kartika, the gap is a full lunar month rather than a day.
What is an ayanamsa?
The offset between the tropical zodiac, which is tied to the equinox, and the sidereal zodiac, which is tied to the fixed stars. Sidereal longitude is tropical longitude minus the ayanamsa. Different schools use different values, and the two compared here differ by 1.1331 degrees.
Which Diwali date was correct in 2020?
14 November 2020, following the new moon of 15 November. That matches the Lahiri computation and the panchang convention used across India. A naive computation under True Pushya Paksha returns 16 October, which is why this page exists.
Is Diwali 2026 affected?
No. The November 2026 new moon puts the Sun at 203.83 degrees under True Pushya Paksha and 202.70 under Lahiri. Both are well inside Libra, so both systems select the same new moon and Diwali falls on 8 November 2026.
Which ayanamsa does Tempora use?
PVRN Rao True Pushya Paksha, stated on every page. For festival dates we follow observed convention where it diverges, and we state both readings rather than picking one silently.
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This page was first published on 11 September 2026 and last updated on 11 September 2026. It documents computed new moon instants and sidereal Sun positions under two ayanamsa systems. A muhurat is personal and no birth chart is computed here. Not financial, medical or personal advice.