The 24 Solar Terms and the Month Pillar: The Day the Month Changes

The saju calendar follows the movement of the sun

The 24 solar terms divide the sun's path across the sky (the ecliptic) into twenty-four sections of 15 degrees each. Because they are set by the sun's position, they fall on almost the same solar-calendar dates every year but drift widely against lunar-calendar dates. They have long served as a farming calendar that tells when the seasons turn, and in saju these terms are the boundaries between months and years.

The twenty-four terms alternate between "jeol" (節) and "junggi" (中氣). The twelve that open the door of a season, such as Ipchun (입춘, Start of Spring), Gyeongchip (경칩) and Cheongmyeong (청명), are jeol, and the twelve that sit between them, such as Usu (우수), Chunbun (춘분) and Gogu (곡우), are junggi. Of these, only the jeol change the month in saju.

The twelve jeol and the month branch

From the moment of Ipchun until just before Gyeongchip is the 寅 month, from Gyeongchip until just before Cheongmyeong is the 卯 month, and so on: each jeol opens one month. So the first month of the saju begins not on 1 January in the solar calendar, and not on the first day of the first lunar month either, but at Ipchun. The "date range" in the table below gives the dates on which each jeol fell in Seoul from 1950 to 2050, and the two right-hand columns are the actual entry times in 2027 and the month pillar for that month.

As the table shows, Sohan (소한) arrives in January in the solar calendar but in saju it is still the last month (丑 month) of the previous year. The 丑 month of 2027 begins at 04:54 on 6 January 2028 and lasts until just before Ipchun 2028, and throughout that time the year pillar remains 丁未 of 2027. In other words, there is a stretch of a little over a month in which the solar-calendar year has changed but the saju year has not.

Junggi do not change the month

Junggi come around the middle of each month. The days that mark the middle of a season, such as the spring and autumn equinoxes (춘분, 추분) and the summer and winter solstices (하지, 동지), are all junggi. Their names are familiar and the change in the weather is clear, so it is easy to mistake them for the day the month changes, but the month pillar stays the same after a junggi passes. For example, everyone born around the 2027 spring equinox (21 March) belongs to the 卯 month that began at Gyeongchip.

This does not mean junggi are never used in saju calculation. The lunar-calendar rule for deciding leap months uses junggi, and some schools look at how many days have passed since a jeol when judging which of the energies hidden in the month branch is in force. Halmeoni does not use this day-counting. Either way, the only boundary that changes the characters of the month pillar is the jeol. Note also that the "junggi" that comes up when explaining hidden stems (지장간) is a different use of the word, referring to the order of the hidden characters inside a branch, and has nothing to do with the junggi of the solar terms.

The year changes at Ipchun

The year pillar also follows the solar terms. A saju year starts at Ipchun and ends just before the next Ipchun. It does not change on 1 January or on Seollal (Lunar New Year). For example, Seollal in 2026 is on 17 February in the solar calendar, but Ipchun comes earlier, on 4 February. So someone born on 10 February 2026 is already a 丙午 (byeong-o) year person, a Horse, even though it is before Seollal.

Halmeoni converts the moment of birth to universal time and compares it with the exact moment of Ipchun calculated on the same basis to decide the year pillar. Because it compares times to the minute rather than comparing dates, someone born on the day of Ipchun lands in one year or the other depending on the hour of birth. The month is decided in the same way, by comparison with the times of the twelve jeol. No longitude correction is applied when deciding the year and month pillars. The longitude correction is used only to set the day and hour to the height of the sun at the place of birth, and the entry of a solar term is the same single moment everywhere on Earth.

There is also a view that takes the winter solstice (동지) as the start of the year, and the folk habit of treating the zodiac animal as changing at Seollal. Halmeoni uses Ipchun as the boundary, as is most widely done in myeongni studies (명리학) today.

Why 3 February and 4 February births can fall in different years

Ipchun arrives around 4 February each year, but the date is not fixed. Between 1950 and 2050, by Seoul time, Ipchun fell on one of 3, 4 or 5 February. Up to 1984 it fell on 5 February about once every four years, and from 2021 it falls on 3 February once every four years. Even for the same "born on 4 February," depending on which year it was, a person may belong to the previous year or to the new year.

• 1984: Ipchun entered at 00:18 on 5 February. So someone born in the daytime on 4 February 1984 has the year pillar 癸亥 (gye-hae), a Pig, and not the Rat (甲子) of 1984.
• 2021: Ipchun entered at 23:58 on 3 February, just before midnight on 3 February. Someone born at 22:00 on 3 February still has the year pillar 庚子 (gyeong-ja), and someone born in the daytime the next day has 辛丑 (sin-chuk).

A worked example: Ipchun day 2027 in Seoul

Ipchun 2027 enters at 10:46 on 4 February. Here is what Halmeoni calculates for two people born in Seoul on that day, at 09:00 and at 12:00. Also listed is a person born at 12:00 the day before, on 3 February.

The person born at 09:00 was born before the moment of Ipchun, so the year pillar is 丙午 and the month pillar is 辛丑. They are still in the last month (丑 month) of 2026. The person born at 12:00 was born after Ipchun, so the year pillar changes to 丁未 and the month pillar to 壬寅. The two were born three hours apart and share the same day pillar, 甲寅, but two pillars, four characters, differ: the year pillar and the month pillar. The month branch has changed from 丑 (winter earth) to 寅 (spring wood), so the season is read differently too, and the strength of the Day Master and the structure (격국) may also change.

If you enter only 4 February 2027 without knowing the time of birth, Halmeoni calculates on the basis of 12:00 and sets the year pillar to 丁未 and the month pillar to 壬寅. Because a solar term falls on the day of birth, it also shows a note on the chart card saying that knowing the time would make the year and month pillars certain. If you were born on the day of Ipchun or of another term, be sure to check the time of birth.

Wolgeon (월건): the year's stem sets the month's stem

The month branch is set by the solar terms, while the month stem (월간, the upper character of the month) is set by looking at the stem of that year. For each year the stem of the 寅 month is fixed, and from there the months advance one step at a time in the order of the 60 gapja. The stems are grouped into five pairs that use the same rule, so after five years the month pairs return to the start. The table below lists the month pillar of each month from 2024 (甲) to 2028 (戊), taken from Halmeoni's calculation.

Old books memorised this as "in a gap or gi year, begin with byeongin" (갑기지년 병인두). It is also explained by the principle of stem combination (천간합). When 甲 (gap) and 己 (gi) combine they become earth, and the yang stem of the fire that gives birth to earth is 丙, so the 寅 month becomes 丙寅. 乙 (eul) and 庚 (gyeong) become metal and begin with 戊, 丙 (byeong) and 辛 (sin) become water and begin with 庚, 丁 (jeong) and 壬 (im) become wood and begin with 壬, and 戊 (mu) and 癸 (gye) become fire and begin with 甲. This is also why the 寅 month of the 丁未 year 2027 is 壬寅.

The 丑 month falls in January of the following year in the solar calendar, but it follows the year stem of the previous year. That is why the 丑 month of the 丁未 year 2027 (January 2028) is 癸丑. Since the year pillar has not changed, the month stem keeps using the previous year's rule.

Jeol are also used to calculate Luck Cycles

The jeol are also the reference when counting the age at which a Luck Cycle (대운) begins. If the cycle runs forward, the time from birth to the next jeol is measured; if it runs backward, the time back to the jeol just before; and three days count as one year. Junggi are not used here either. The closer the jeol in the counting direction is to the moment of birth, the earlier the first Luck Cycle begins.