Appearance
Four Pillars (四柱八字)
The Four Pillars encode a moment in time as four stem-branch pairs — one each for year, month, day, and hour. They form the foundation of 命理學 (Chinese fate analysis).
The Sexagenary Cycle (六十甲子)
10 Heavenly Stems (天干) cycle with 12 Earthly Branches (地支), producing 60 unique combinations:
Stems: 甲 乙 丙 丁 戊 己 庚 辛 壬 癸
Branches: 子 丑 寅 卯 辰 巳 午 未 申 酉 戌 亥
The cycle starts at 甲子 and repeats every 60 units (LCM of 10 and 12).
Year Pillar (年柱)
The year pillar changes at 立春 (Start of Spring, solar longitude 315°), not January 1st. stembranch computes the exact 立春 moment using VSOP87D to determine which sexagenary year a given date falls in.
Formula: (year - 4) mod 60 gives the index into the 60-year cycle, with adjustment at the 立春 boundary.
Month Pillar (月柱)
The month changes at each 節 (major solar term) — the odd-numbered solar terms that mark the start of each solar month:
| Solar Month | 節 (Start) | Branch |
|---|---|---|
| 1 | 立春 (315°) | 寅 |
| 2 | 驚蟄 (345°) | 卯 |
| 3 | 清明 (15°) | 辰 |
| 4 | 立夏 (45°) | 巳 |
| 5 | 芒種 (75°) | 午 |
| 6 | 小暑 (105°) | 未 |
| 7 | 立秋 (135°) | 申 |
| 8 | 白露 (165°) | 酉 |
| 9 | 寒露 (195°) | 戌 |
| 10 | 立冬 (225°) | 亥 |
| 11 | 大雪 (255°) | 子 |
| 12 | 小寒 (285°) | 丑 |
The month stem is determined by the year stem using the 五虎遁 (Five Tigers) rule:
| Year Stem | Month 1 (寅) Stem |
|---|---|
| 甲, 己 | 丙 |
| 乙, 庚 | 戊 |
| 丙, 辛 | 庚 |
| 丁, 壬 | 壬 |
| 戊, 癸 | 甲 |
Day Pillar (日柱)
The day pillar is purely arithmetic — independent of astronomy. It cycles through the 60 sexagenary pairs with a fixed epoch:
dayIndex = (JDN + 49) mod 60where JDN is the Julian Day Number at midnight UT. The day boundary is midnight (00:00 local time), consistent with modern Chinese calendar practice.
Hour Pillar (時柱)
Each day is divided into 12 two-hour periods (時辰):
| Branch | Hours | Name |
|---|---|---|
| 子 | 23:00–01:00 | Early Rat |
| 丑 | 01:00–03:00 | |
| 寅 | 03:00–05:00 | |
| ... | ... | |
| 亥 | 21:00–23:00 |
The hour stem is determined by the day stem using the 五鼠遁 (Five Rats) rule, analogous to the Five Tigers rule for months.
True solar time: stembranch optionally computes hour pillars using true solar time (via the Equation of Time from VSOP87D), which can shift the hour boundary by up to ±16 minutes.
API
typescript
import { computeFourPillars } from 'stem-branch';
const pillars = computeFourPillars(new Date(2024, 1, 10, 14, 30));
// → {
// year: { stem: '甲', branch: '辰' },
// month: { stem: '丙', branch: '寅' },
// day: { stem: '壬', branch: '午' },
// hour: { stem: '丁', branch: '未' }
// }