How the Four Pillars Are Determined: From a Birth Time to Eight Characters

Year Pillar: Boundary at the Start of Spring

The change of the year pillar in bazi is determined by the solar term "start of spring" (lichun), not by the first day of the lunar year (zhengyue chuyi).

Specifically, it is based on whether the birth moment has passed the lichun moment of that year. If born before the lichun moment, even if after the lunar New Year, the year pillar still belongs to the previous year. For example, the start of spring in 2023 falls on February 4 in the Gregorian calendar. If someone is born on February 3, their year pillar is still that of the Renyin year (2022), not the Guimao year (2023).

Therefore, to determine the year pillar, the first step is to find the exact lichun moments for the birth year and the previous year.

Month Pillar: Boundary at the "Jie" Solar Terms

The change of the month pillar is based on the twelve "jie" (node) solar terms within the twenty-four solar terms, not on the first day of each lunar month. These twelve jie in sequence are: lichun (start of spring), jingzhe (awakening of insects), qingming (clear and bright), lixia (start of summer), mangzhong (grain in ear), xiaoshu (minor heat), liqiu (start of autumn), bailu (white dew), hanlu (cold dew), lidong (start of winter), daxue (major snow), and xiaohan (minor cold).

The arrival of each "jie" marks the change of the earthly branch for the month pillar. For instance, births between lichun and jingzhe have the earthly branch "yin" for the month pillar; births between jingzhe and qingming have the earthly branch "mao." Thus, leap months in the lunar calendar do not affect the month pillar, which is entirely determined by the solar terms.

As for the heavenly stem of the month pillar, it is derived from the heavenly stem of the year pillar using a fixed mnemonic formula (the "month from year" method). For example, if the year's heavenly stem is Jia or Ji, the heavenly stem for the first month (yin month) is Bing; if the year's stem is Yi or Geng, the first month's stem is Wu, and so on. This derivation process follows fixed rules and is independent of the specific birth year.

Day Pillar: Continuous Cycle of the Sexagenary Cycle

The day pillar is not affected by the rules for changing the year and month pillars. It operates independently as a continuous sexagenary cycle (liushijiazi), running day after day from ancient times without interruption due to changes in year or month.

Therefore, to determine the heavenly stem and earthly branch for a given day, there is no shortcut; one must consult a ten-thousand-year calendar (wannianli) or reliable astronomical data to confirm the corresponding sexagenary number for that day. This is the most fundamental and crucial step in charting.

Hour Pillar: Determined by the Day's Heavenly Stem

The calculation of the hour pillar depends on the heavenly stem of the day pillar. This uses the "five rats hide into time" mnemonic (also known as the "hour from day" method), with the following rules:

• If the day's heavenly stem is Jia or Ji, the heavenly stem for the zi hour (23:00-00:59) is Jia.
• If the day's stem is Yi or Geng, the zi hour's stem is Bing.
• If the day's stem is Bing or Xin, the zi hour's stem is Wu.
• If the day's stem is Ding or Ren, the zi hour's stem is Geng.
• If the day's stem is Wu or Gui, the zi hour's stem is Ren.

Starting from the heavenly stem of the zi hour, following the order of heavenly stems (Jia, Yi, Bing, Ding...) and corresponding them to the twelve two-hour periods (zi, chou, yin, mao...), one can determine the heavenly stem for each time period. The earthly branch for the hour pillar is fixed as the branch corresponding to the birth time period (e.g., 23:00-00:59 is zi time, so the earthly branch is zi).

From this, it is clear that if the day pillar is calculated incorrectly, the hour pillar will inevitably be wrong as well.

Special Handling of the Zi Hour

The zi hour (23:00 to 00:59 of the next day) spans two days. In traditional charting, there are two different approaches regarding whether the "late zi hour" (23:00-23:59) belongs to the current day or the next day. This is a historical and academic point of divergence.

In designing the charting function for this website, this is set as an option that users can choose for themselves. After users input their birth time, the system calculates step by step based on the selected zi hour handling rule and the above rules for determining the year, month, day, and hour pillars. It then lists the basis for each pillar's determination (e.g., which lichun moment, which solar term, the day's stem and branch, the selected zi hour rule, etc.) and ultimately presents the complete bazi.