Twenty-four points on the Sun’s path
The 24 solar terms divide the Sun’s apparent yearly path into 15° steps. They alternate between “jie” (节, sectional terms) and “qi” (中气, principal terms). The principal terms include the equinoxes and solstices; the sectional terms fall halfway between them. Because they track the Sun, they fall on nearly the same Gregorian dates every year, give or take a day. The Hong Kong Observatory publishes their exact times each year, and we test our calendar engine against its tables for every year from 1901 to 2100 (see the method page).
San Ming Tong Hui explains the names of the terms one by one and sums up the year in two halves:
大抵合而言之，上半年主长生，曰雨曰雷曰风，皆生之气，下半年主生成，曰露曰霜曰雪，皆成之气。“Taken together: the first half of the year governs growth — rain, thunder and wind are all energies of birth; the second half governs completion — dew, frost and snow are all energies of maturity.”— San Ming Tong Hui 《三命通會》, 卷二 · 论四时节气. Original text: Wikisource · also on ctext.org (different edition). Translation: bazimethod.
The Bazi year starts at 立春
The year pillar changes at the exact moment of 立春 (Start of Spring, the Sun at 315° ecliptic longitude), around 4 February. It does not change on 1 January, and it does not change at the Lunar New Year, which can fall anywhere from about 21 January to 20 February. Someone born on 1 February 2025 has the 甲辰 year pillar (2024’s) even though the Lunar New Year of 2025 fell on 29 January; someone born on 4 February 2025 has 乙巳.
This is the single most common source of disagreement between quick zodiac tools (which often use Lunar New Year) and Bazi calculators. Our calculator uses 立春 to the minute and warns you if you were born within two hours of it.
Twelve jie begin the twelve months
Only the twelve sectional terms (jie) matter for the month pillar. Each starts a Bazi month, named by an earthly branch:
| Jie term | English | Approx. date | Month branch begins |
|---|---|---|---|
| 立春 | Start of Spring | ~4 Feb | 寅 |
| 惊蛰 | Awakening of Insects | ~6 Mar | 卯 |
| 清明 | Clear and Bright | ~5 Apr | 辰 |
| 立夏 | Start of Summer | ~6 May | 巳 |
| 芒种 | Grain in Ear | ~6 Jun | 午 |
| 小暑 | Minor Heat | ~7 Jul | 未 |
| 立秋 | Start of Autumn | ~8 Aug | 申 |
| 白露 | White Dew | ~8 Sep | 酉 |
| 寒露 | Cold Dew | ~8 Oct | 戌 |
| 立冬 | Start of Winter | ~7 Nov | 亥 |
| 大雪 | Major Snow | ~7 Dec | 子 |
| 小寒 | Minor Cold | ~6 Jan | 丑 |
San Ming Tong Hui describes the months in this way when it lists the hidden stems: “the first month is established in 寅” (正月建寅), the second in 卯, and so on.
此十二支按十二月各藏五行为人元，以配四时则春暖秋凉冬寒夏热，如环无端，终而复始，岁功毕而成一年。“These twelve branches, following the twelve months, each store the five elements as the human origin; matched to the four seasons — warm spring, cool autumn, cold winter, hot summer — they go round like a ring without end, finishing and beginning again; when the year’s work is done, a year is complete.”— San Ming Tong Hui 《三命通會》, 卷二 · 论人元司事. Original text: Wikisource · also on ctext.org (different edition). Translation: bazimethod.
Month stems: the “five tigers” rule
The month branch is fixed by the solar term, but which stem sits on it? The answer depends on the year stem. The 寅 month of a 甲 or 己 year is always 丙寅; of an 乙 or 庚 year, 戊寅; and so on. San Ming Tong Hui preserves the mnemonic:
甲已之年丙作首，乙庚之岁戊为头，丙辛之岁寻庚上，丁壬壬位顺行流，更有戊癸何处起？甲寅之上好追求。“In Jia or Ji years, Bing comes first; in Yi or Geng years, Wu is at the head; in Bing or Xin years, look for Geng; in Ding or Ren years, flow onward from Ren; and where do Wu and Gui years begin? Seek it at Jia-Yin.”— San Ming Tong Hui 《三命通會》, 卷二 · 论遁月时. Original text: Wikisource · also on ctext.org (different edition). Translation: bazimethod.
(The transcription writes 已 for 己 in the first phrase; we quote it as transcribed.) After the first month, the stems simply continue in order: a 丙寅 first month is followed by 丁卯, 戊辰 and so on.
Hour stems: the “five rats” rule
The hour stem works the same way from the day stem. On a 甲 or 己 day the 子 hour is 甲子; on an 乙 or 庚 day it is 丙子; and so on:
古歌曰：甲己还加甲，乙庚丙作初，丙辛从戊起，丁壬庚子居，戊癸何方发？壬子是直途。“The old song says: Jia and Ji return to Jia; Yi and Geng begin with Bing; Bing and Xin start from Wu; Ding and Ren dwell at Geng-Zi; from where do Wu and Gui set out? Ren-Zi is the straight road.”— San Ming Tong Hui 《三命通會》, 卷二 · 论遁月时. Original text: Wikisource · also on ctext.org (different edition). Translation: bazimethod.
This rule creates a subtle point for births between 23:00 and midnight. The 子 hour starts at 23:00, but the day pillar traditionally changes at midnight in one school and at 23:00 in another. Both schools agree that the hour stem from 23:00 follows the next day’s stem. Our calculator lets you choose which day pillar to use; see the method page.
The same instant everywhere
A solar term is an astronomical instant, the same moment worldwide. 立春 2025 occurred at 22:10 Hong Kong time on 3 February, which was 14:10 UTC and 09:10 in New York. A baby born at 08:30 that morning in New York — 21:30 in Hong Kong, still 3 February there too — was born before 立春 and has the 甲辰 year; one born at 09:30 has 乙巳. We compute year and month pillars from the absolute instant, which is why your time zone and its historical daylight-saving rules matter even for the year pillar near a boundary.
Solar terms and luck pillars
The distance from birth to the nearest jie term, forward or backward, sets the age at which the first luck pillar begins, by the rule of three days to one year.
Common questions
Why does my Chinese zodiac animal differ between websites?
Because the zodiac year is defined in two ways in popular use. Bazi and most fate traditions change the animal at 立春; everyday Chinese culture and many zodiac websites change it at the Lunar New Year. For anyone born between late January and 4 February, the two methods can disagree. For a Bazi chart, 立春 is the rule.
Are solar terms the same every year?
Nearly. They fall on the same Gregorian date or one day either side, because the Gregorian calendar itself is designed to follow the Sun. Their exact times shift by about six hours each year and jump back after each leap day.
Does the middle term (qi) matter for anything?
Not for the pillars. The twelve principal terms matter for the lunar calendar (they decide leap months) and are used by some practitioners for finer seasonal judgements, but the year and month pillars change only at the twelve jie terms.
Check your chart
If you were born in early February, early in any Gregorian month (between about the 4th and 8th), or near midnight, check the warnings in the calculator’s output: it tells you how close you were to a solar term and what would change if your birth time were off.
Keep learning
Classical quotations are copied from the public-domain texts on Wikisource; links let you check every line. English translations are our own. Traditional Bazi statements are reported as tradition, not as fact.