24 Solar Terms & Saju

Two children are born in the early hours of February 4th. Their birth times are two minutes apart. Same hospital, same day, same dark morning.

Yet one belongs to the Eulsa year and the other to the Byeongo year. Their months differ as well. Two minutes apart on the same morning — why would the year split?

The short answer: a Saju month turns on a solar term

The position that carries the month in a chart is called the month pillar. It does not turn on the first of a calendar month. It turns at the instant a solar term enters. So a birth on February 1st can still belong to January, and a birth on March 3rd can still belong to February.

The same holds for the year. A Saju year does not begin on January 1st; it begins at Ipchun. Ipchun is the one solar term where the year boundary and the month boundary sit in the same place, so both pillars move across that single moment.

What the solar terms actually measure

A solar term is a mark on the sun's path. The ecliptic is divided into 24 parts, and a name is given every 15 degrees of the sun's travel. So solar terms are not lunar at all — they are solar. That is why their dates land on nearly the same spot in the Western calendar each year.

Because the names describe weather, people often file them under seasonal folklore. In Saju they are not seasonal decoration. They are the ruling marks of the calendar: they decide which slot each of the twenty-two stems and branches falls into. If the characters themselves are unfamiliar, start with Heavenly Stems and Earthly Branches.

Only twelve of the twenty-four are boundaries

Here is a widely repeated line that needs correcting. "The 24 solar terms change the month pillar" is not accurate. The 24 alternate between twelve jeol (節) and twelve junggi (中氣), and only the twelve jeol move the month.

The familiar names — the equinoxes, the solstices, Usu — are mostly junggi. A birth on one of those days does not move the month pillar. Because jeol and junggi alternate roughly every fifteen days, a calendar month contains two solar terms but only one boundary.

The boundary is a time, not a date

Term entry is given to the minute. There is an exact instant when the sun reaches that point, and the month turns on it. Which means two people born on the same date can land on opposite sides.

Below are the twelve month boundaries of 2026. Times are Korea Standard Time, and the right column is the month pillar that begins at that instant.

Ipchun, where the year and the month turn together

Ipchun 2026 falls on February 4th at 05:02. Put two births two minutes on either side of that instant into the engine and they split like this.

The year and the month moved together. The day pillar, though, is Giyu for both. A day boundary is set by midnight, not by a solar term. That is the answer to the question this article opened with.

Why nothing moves at a junggi

A junggi marks the middle of a month rather than its edge. It is a milestone, not a boundary. Usu 2026 falls on February 19th at 00:51, and births on either side of it keep the same Gyeongin month pillar. The spring equinox behaves the same way.

This does not make junggi useless. Because they mark the heart of a season, they help gauge how far a season's energy has ripened. They simply do not change a pillar.

A different problem from time correction

Reading about term boundaries, you will run into true solar time and the midnight-hour debate. They are easy to confuse, but they move different pillars.

Term boundaries move the month and the year. Time correction moves the hour pillar. If you want to know how a recorded birth time is converted, Birth Time in Saju covers exactly that.

Your luck cycle's starting age comes from here too

Solar terms do not stop at fixing the chart. The age at which your ten-year Daeun cycle begins also comes out of them: the distance in days from your birth to the nearest term entry, forward or backward, is converted into an age.

So an imprecise entry time shakes the starting age as well. Which direction a Daeun runs and when it starts is shown with real calculations in Daeun and Seun.

Season is a condition, not a conclusion

The term you were born under is a condition for reading a chart. Born in spring means born while Wood is at its strongest; born in autumn means Metal is. That condition matters as background when you weigh the balance of the whole chart.

But a season does not decide a personality. Claims like "born in spring, therefore enterprising" rest on very little. How to handle a seasonal imbalance belongs to the useful-god discussion, so we leave it to the Yongsin guide.

Myth one: counting the month from the solar birthday

Born in March, therefore a March chart — it is a common shortcut. But because a solar term rules the Saju month, births from March 1st to around March 5th still belong to the previous month. Anyone born in the first week of a month should check.

Myth two: the year turns on January 1st

Many people assume the zodiac animal changes on New Year's Day. In Saju the year turns at Ipchun. Births in January and early February often belong to the previous year. It is also not the Lunar New Year: that follows the moon, while a Saju year follows the sun.

Four sentences for checking your own term

It looks fussy, but it is four checks in order. The right column is the judgment people usually get wrong at that step.

In one sentence

Which solar term your birthday falls inside can be read from the month pillar on your result screen. If you were born in early February, check the year pillar alongside it.

Check my solar term

What we checked

Entry times were compared against Korea Astronomy and Space Science Institute reference values. Ten of the twelve 2026 boundaries matched to the minute; at Gyeongchip and Cheongmyeong our calculation ran one minute late. We printed that under the table rather than hiding it.

The month pillars were verified separately from the times. One of the entry tables we checked against carried correct times but stem-branch labels from a different year, so we recounted them with the Ohodun rule. 2026 is a Byeongo year, which makes the In month Gyeongin, and the Chuk month just before Ipchun Gichuk under the preceding Eulsa year.

The charts on either side of the boundary are direct output from our own engine. If Saju is new to you, start with What Is Saju, and keep the Saju glossary open for the vocabulary.