Methodology and accuracy
Every result on this site has a rule behind it. This page states the rules, the sources and how far each part has been verified, including what is still uncertain.
1. General principles
- Calculation is separated from interpretation. Lunar dates, Can Chi, solar terms and star positions are deterministic and testable. The meaning of a good day, compatibility or a star is cultural belief, for reference only.
- Everything runs in your browser; birth data is not sent to a server.
- Variants are stated. Schools of day selection and Tu Vi differ in places; when we pick one, we say which and why.
2. The lunar calendar
The Vietnamese lunar calendar is an astronomical problem, not a fixed lookup table. We compute it directly in UTC+7:
- The first day of a month is the day containing the new moon. New-moon times use Jean Meeus's full method from Astronomical Algorithms (chapter 49: periodic terms and planetary corrections) with a ΔT correction.
- Month 11 is the month containing the winter solstice. The Sun's longitude uses Meeus's approximate formula.
- A leap month is a month with no mid-term (zhōngqì); such a year has 13 months.
3. Verified accuracy
There is no official Vietnamese table to copy, so we verify against an independent table:
| Check | Result |
|---|---|
| Every day 1901–2099 (72,723 days) against the Chinese lunar table 1900–2099 (source data from the Hong Kong Observatory, via the python-lunardate library). The Chinese calendar uses UTC+8, so we run the same algorithm at UTC+8. | No mismatching day from 1979 to 2099. 180 days differ across six historical years 1914–1978 (different time-zone conventions); none affect the future. |
| Tết, leap months, Mid-Autumn and Hùng Kings' day for 2020–2028 against known dates. | All match (part of the automated tests). |
| Sanity for every day 2025–2099: lunar day always 1–30 and continuous; Gregorian ⇄ lunar conversion is consistent both ways. | Pass. |
Not verifiable: the Vietnamese and Chinese calendars differ in some years because of the time zone (for example 2030, 2031, 2032, 2034, 2037–2041…). We have no official Vietnamese table to check that difference; we rely on the same astronomy at UTC+7, which is the definition of the Vietnamese lunar calendar. For days where the new moon falls within minutes of midnight, results may differ from a printed calendar that used different precision. For important dates, cross-check with another source.
4. Can Chi, solar terms, Hoàng đạo
- Can Chi. Years, months, days and hours are named by a cycle of 10 stems × 12 branches (60 combinations). The day cycle is continuous, anchored on 1 Jan 2000 = Mậu Ngọ. See Heavenly Stems and Earthly Branches.
- Solar terms. The 24 terms mark every 15° of the Sun's longitude; a term begins on the day the Sun reaches that longitude (Vietnam time). See The 24 solar terms.
- Hoàng đạo / Hắc đạo. Twelve stars (Thanh Long … Câu Trần) started from the day's branch and the solar-term month; auspicious hours depend on the day's branch. See Auspicious days.
5. Day selection and compatibility
The day finder adds and subtracts points for traditional factors: Hoàng đạo/Hắc đạo (±2), the twelve day officers (Thập nhị trực, −2 to +2 depending on the activity), Tam nương days (−3), Nguyệt kỵ (−2), Dương công kỵ nhật (−4, avoid) and, for your birth year, a clashing day (−4, avoid) or harmonious day (+1). It also flags Kim Lâu and Tam tai years.
Compatibility considers Earthly Branch (triad or six-harmony +2, clash −3, harm −2), Heavenly Stem (combine +1, clash −1), nạp âm element (generating +2, same +1, overcoming −2) and Bát trạch cung phi (same East/West group +1, different −1).
Note: the rules (Tam nương days, clashing years…) are traditional and mechanically tested, but the point weights are ours, not taken from a specific book, and the day-officer table per activity differs between books. Scores only compare days with each other; they are not a "probability of success".
6. Tu Vi birth chart
We place stars following the Vietnamese Tu Vi tradition (Thiên Lương school): Life and Body palaces, the 12 palaces, Cục, the 14 main stars, auxiliary and minor stars, the Thái Tuế, Lộc Tồn and Tràng sinh rings, Tuần, Triệt, Tứ hóa, great limits, small limits, and star brightness for most stars.
| Part | Verification |
|---|---|
| Life/Body palaces, 12 palaces, Cục, main and auxiliary stars, Thái Tuế and Lộc Tồn rings, Tuần, Triệt, Tứ hóa, great and small limits (about 90 stars) | Compared with an open-source Vietnamese Tu Vi implementation (lasotuvi, MIT) on 6,000 random cases: identical except the two known differences below. |
| Thai and Dưỡng in the Tràng sinh ring | The reference puts them in reverse of the standard order (Tuyệt → Thai → Dưỡng). We keep the standard order. |
| Hỏa Tinh and Linh Tinh for years of Sửu, Tỵ, Dậu | The reference and we use different starting points. We keep ours; an expert should confirm. |
| Star brightness table | Transcribed from one source; Thiên Phủ has two variants and we chose the majority of Vietnamese references. Other books may differ in some cells. |
| Year outlook (lưu niên) | Not checked against an independent implementation; only internal consistency tests. |
The chart page shows star positions and a short symbolic meaning of the main stars in the Life palace. We do not give full readings because no expert has reviewed them.
7. Limits and open questions
- Late Tý hour (23:00–24:00) is counted on the same birth day; some schools use the next day.
- Births after the 15th of a leap month count toward the following month; some schools differ.
- People born abroad should convert their birth time to Vietnam time themselves.
- Cung phi uses the lunar year; some sources use the start of spring (Lập xuân).
- The articles are written from public sources and not yet reviewed by an expert.
Sources
- Jean Meeus, Astronomical Algorithms (new-moon and solar-longitude formulas).
- Espenak & Meeus, ΔT polynomials (NASA Eclipse Web Site).
- python-lunardate (lunar table 1900–2099 used for comparison).
- lasotuvi (open-source Tu Vi implementation, used for comparison and as the source of the brightness table; MIT license).
Found a mistake? Tell us and include a source for comparison. The calculation code is tested automatically on every change.