Dwadashamsha, also written Dwadasamsa, is the twelfth divisional chart or D12. A sidereal 30° sign is split into twelve parts of 2°30′; the mapped signs proceed in zodiacal order from the natal sign itself. This page explains exactly which convention the calculator uses and how to check a placement by hand.
The sign assignment in Parāśara's verse
Bṛhat Parāśara Horā Śāstra, chapter 6, verse 15 describes reckoning each D12 division from the same natal rāśi. Unlike D9 or D10, there is no odd-sign and even-sign switch in this rule. Count the D1 sign as part one, the next zodiac sign as part two, and continue forward until part twelve reaches the preceding sign.
| D1 position | Part | D12 sign |
|---|---|---|
| 0°–<2°30′ Gemini | 1 | Gemini |
| 10°–<12°30′ Gemini | 5 | Libra |
| 17°30′–<20° Scorpio | 8 | Gemini |
| 27°30′–<30° Pisces | 12 | Aquarius |
The Gemini 11° and Scorpio 19° examples also appear in P. V. R. Narasimha Rao, Vedic Astrology: An Integrated Approach, §6.2.12. They provide a second way to check the arithmetic against a published practitioner's worked examples.
Division names in the source
The same verse gives a four-name sequence for the twelve portions: Ganesha, Ashvini Kumaras, Yama, and Sarpa, repeating three times. The calculator prints the matching name beside each point to make the verse's sequence visible. These are textual division names. A deity label alone does not establish an interpretation of a person's parents or ancestors.
| Parts | Presiding name |
|---|---|
| 1, 5, 9 | Ganesha |
| 2, 6, 10 | Ashvini Kumaras |
| 3, 7, 11 | Yama |
| 4, 8, 12 | Sarpa |
Calculate a D12 placement by hand
Find the D1 sidereal longitude and the degree within its 30° sign. Divide that within-sign degree by 2.5 and take the integer part, from zero to eleven. Advance that many signs from the D1 sign. Thus 11° Gemini has zero-based part four: four signs after Gemini is Libra. Nineteen degrees of Scorpio has zero-based part seven: seven signs after Scorpio is Gemini.
At 2°29′59″ Aries the assignment is still Aries. At exactly 2°30′ Aries the second part begins in Taurus; at exactly 5° Aries the third part begins in Gemini. A position at 1°15′ Aries is halfway through the first part, displayed as 15° Aries in the derived D12 coordinate. At exactly 30° Aries, D1 is Taurus and D12 restarts in Taurus.
With zero-based sign index s and within-sign degree d, the part index is p = floor(d / 2.5), the D12 sign index is (s + p) modulo 12, and the displayed D12 degree is 12 × (d − 2.5p). This is a coordinate transform of D1, not a new observation of the sky.
Reading the D12 with the natal chart
Rao's textbook lists D12 for matters concerning parents and the blood relatives of parents. That is a subject area in Jyotish practice, not an automatic conclusion from a single sign. A traditional reading compares D12 with the rāśi chart and its context. The calculator supplies the positions and makes no claim about a family member, relationship, or event.
The D12 Lagna is especially sensitive to birth-time errors: 2°30′ of the D1 ascendant spans a whole D12 sign. The tool flags points within 0.1° of a boundary. If the recorded time, historical UTC offset, location, ayanamsha, node convention, or ephemeris differs, placements may differ. An unknown birth time cannot support the D12 Lagna shown here.
This site's D1 uses Astronomy Engine 2.1.19 with an approximate Lahiri ayanamsha and mean lunar nodes; it does not use Swiss Ephemeris or infer historical time-zone rules. The local birth-date input covers 1800–2100, with an extra warning if a converted UTC instant falls outside the lunar-node range.
References
- Bṛhat Parāśara Horā Śāstra 6.15 — published Sanskrit and translation of the same-sign starting rule and repeating four division names.
- P. V. R. Narasimha Rao, Vedic Astrology: An Integrated Approach — §6.2.12, two worked D12 examples, and the textbook's table of D12 subject matter.
- Astronomy Engine 2.1.19 — the astronomy library behind this site's sidereal D1 calculation.