Solar Term Moments: Timing of the Twenty-Four Solar Terms

The twenty-four solar terms divide the solar year according to the sun’s position on the ecliptic. Each term occupies fifteen degrees of celestial longitude. 立春 is defined not as a calendar day but as the moment when the sun reaches 315 degrees of ecliptic longitude. The same principle applies to every other solar term: the term begins at an exact point in the sun’s apparent motion, not at midnight, not at the start of a lunar month, and not at the start of a Gregorian month.

节气是黄经度数，不是日历上的一天

A solar term is best understood as a degree reading on the ecliptic rather than as a date on a wall calendar. The term 交节 refers to the instant at which the sun crosses the boundary between two fifteen-degree sectors. In the cycle of the twenty-four terms, these boundaries occur at intervals of fifteen degrees. 立春 marks the arrival at 315 degrees, 雨水 at 330 degrees, and so on through the full circle. Because the sun’s apparent speed along the ecliptic is not perfectly uniform, the interval between successive 交节时刻 is not exactly equal in days, though the angular interval is always fifteen degrees. This is why solar terms can appear to shift by a day or two on the Gregorian calendar from year to year.

For bazi purposes, the relevant unit is not the day on which a term falls but the minute at which the sun crosses the boundary. A person born on the day of 立春 has not automatically entered the new annual pillar. The change occurs only at the recorded 交节时刻 for that year. Before that minute, the previous annual pillar and previous monthly pillar still apply. After that minute, the new pillars take effect. The distinction between a solar term as a degree coordinate and a solar term as a printed date is therefore not a technical nicety; it directly determines how a chart is erected.

为什么要精确到分钟

The minute-level precision of 交节时刻 matters because the monthly pillar in bazi changes at that instant. If the time of the solar-term boundary is treated as a full day, a birth in the early hours of that day could be assigned to the wrong month. The lunar calendar does not provide a reliable substitute here. The first day of a lunar month and the first day of a Gregorian month are administrative or calendrical boundaries. The monthly pillar does not follow them. It follows the sun’s position.

A difference of a few minutes across a 交节 can separate one month pillar from another. The same applies to the annual pillar at 立春. When a birth occurs close to the boundary, the recorded birth time must be compared with the recorded 交节时刻 for that location and time zone. If the birth time is before the boundary, the chart belongs to the preceding month. If it is after, it belongs to the new month. This is why the engine stores solar-term data to the minute rather than rounding to the day. A rounded value would silently introduce errors precisely in the cases where the distinction is most consequential.

本站节气数据的覆盖范围

The engine’s solar-term data is stored in minute units and covers the years from 1899 to 2101. Within this range, the engine reads the recorded 交节时刻 for each term and compares it against the birth time supplied by the user. The data is not extrapolated, and the engine does not produce an approximate solar-term time for years outside the supported range. If a year falls before 1899 or after 2101, the engine returns an error rather than guessing.

This range is not presented as a philosophical limit on the solar terms themselves. The terms can be calculated for earlier and later periods by astronomical methods. The stated range is a property of this engine’s dataset: it contains stored values only for those years. The refusal to return an approximation is deliberate. In bazi work, an approximate 交节时刻 can place a birth on the wrong side of a monthly or annual boundary. The engine therefore treats missing data as missing data, not as an invitation to estimate.

出生在交节当天的处理次序

When the birth date is the same as the date on which a solar term occurs, the procedure is sequential. First, retrieve the exact 交节时刻 for that term and year. Second, compare it with the birth time. If the birth time is before the 交节时刻, the chart is assigned to the previous month. If the birth time is after, the chart moves into the new month. There is no special rule that places all births on the solar-term day into the new month, and no rule that keeps them all in the old month. The comparison against the minute is the only determinant.

A second issue can arise when the birth time is also near the 子时 boundary. In that case, the day pillar may change as well as the month pillar. The 子时 division involves its own conventions, and an ambiguous birth time near that boundary can affect both the day and the hour. When both uncertainties are present, the practical procedure is to list both candidate charts rather than forcing a single answer. If the birth time is close to 子时 and close to a 交节, the month pillar and the day pillar may each shift depending on how the time is resolved. Presenting both results as candidates preserves the uncertainty instead of hiding it.

交节与春节不是同一条线

交节 belongs to the logic of the solar year and governs the annual and monthly pillars in bazi. 春节 belongs to the lunar calendar and governs the folk zodiac year. These are two different boundaries and they cannot be substituted for one another. 立春 is the 交节 that changes the annual pillar. 春节 is the first day of the first lunar month. In some years the two fall close together; in others they are separated by days. A birth between 春节 and 立春 may already be in the new folk zodiac year while still belonging to the previous annual pillar, or the reverse may apply.

The distinction matters because the two systems answer different questions. The folk zodiac year is a cultural and calendrical convention tied to the lunar new year. The bazi annual pillar is an astronomical boundary tied to the sun’s arrival at 315 degrees. A chart constructed with 春节 as the annual boundary would be using the wrong line. A folk zodiac statement made with 立春 as the boundary would also be using the wrong line. Each system has its own internal consistency. The error lies in mixing them.

The same separation holds for the monthly pillar. The lunar month begins at the new moon, but the bazi month begins at the 交节 of the relevant solar term. These two boundaries can differ by weeks. A person born on the first day of a lunar month has not necessarily entered a new month pillar. A person born on the day of a solar term has not necessarily entered a new month pillar either. In both cases, the determining fact is the 交节时刻 compared with the birth time, not the lunar date or the Gregorian date.

The reference treatment of 交节时刻 therefore has a single recurring theme: the boundary is exact, it is solar, and it is not a day. The engine’s role is to supply the stored boundary time and to compare it with the birth time. Where a comparison is impossible because the year is outside the dataset, the engine declines to answer. Where the comparison is possible, the result depends on the minute, not on the printed calendar date. This is the standard the present knowledge base applies to all questions involving the transition between annual pillars or monthly pillars.