Solar terms and your saju chart: when the year and month really begin

Twenty-four points on the sun’s path

Seen from Earth, the sun travels a full circle through the sky over a year. The East Asian calendar marks that circle every fifteen degrees, giving twenty-four solar terms. Four of them you already know: the spring equinox (0°), the summer solstice (90°), the autumn equinox (180°) and the winter solstice (270°). Ipchun, the start of spring, sits at 315° — forty-five degrees before the spring equinox.

Because they are defined by the sun’s position and not by a calendar date, each term is an instant: a specific hour and minute, the same everywhere on Earth, shown in whatever clock you happen to keep. That is what makes them computable — and what makes rounding them to a whole day a real error.

Twelve set the months; twelve do not

The terms alternate between two kinds. The twelve “section” terms (절기) open the twelve saju months; the twelve “middle” terms (중기), which include all four equinoxes and solstices, fall inside a month and do not divide anything. So the summer solstice does not start a month — the term before it, mangjong, does.

The saju months run through the animal branches in order, beginning with the Tiger month at ipchun:

• Ipchun 입춘 (~4 Feb) opens the Tiger month 寅 — and the saju year
• Gyeongchip 경칩 (~5 Mar) — Rabbit 卯
• Cheongmyeong 청명 (~5 Apr) — Dragon 辰
• Ipha 입하 (~5 May) — Snake 巳
• Mangjong 망종 (~6 Jun) — Horse 午
• Soseo 소서 (~7 Jul) — Goat 未
• Ipchu 입추 (~7 Aug) — Monkey 申
• Baengno 백로 (~7 Sep) — Rooster 酉
• Hallo 한로 (~8 Oct) — Dog 戌
• Ipdong 입동 (~7 Nov) — Pig 亥
• Daeseol 대설 (~7 Dec) — Rat 子
• Sohan 소한 (~5 Jan) — Ox 丑, the last month of the saju year

The year turns at ipchun — to the minute

The year pillar changes at ipchun rather than on 1 January or at Lunar New Year. And ipchun moves. In 2024 it fell on 4 February at 17:26 Korea Standard Time; in 2025 it came a day earlier, 3 February at 23:10; in 2027 it will be 4 February at 10:46. Go back to 1990 and it was 4 February at 11:13.

So a child born at 22:00 on 3 February 2025 belongs to the Wood Dragon year (甲辰), and a child born ninety minutes later, at 23:30, belongs to the Wood Snake year (乙巳) — different year pillar, different month pillar, different chart. Every other section term draws the same kind of line through its own day, twelve times a year.

Why calculators disagree on boundary days

Most quick tools carry a table of solar-term dates, not instants. On a boundary day they assign everyone the new month, or everyone the old one, and half the people born that day get the wrong year or month pillar without ever knowing it. The chart still looks perfectly plausible; the error only shows when you compare it against an almanac that prints the minute.

Stale tables are a second failure. We know because we caught one in our own stack: an almanac library that advertised support for 2020 to 2030 was returning the identical 2026 table for all eleven years, so ipchun sat at 05:02 on 4 February no matter the year — up to thirteen hours off the true instant. We replaced it with direct astronomical computation the day we found it. If a tool does not say how it gets its terms, that is worth asking.

How ioreum computes them

Our engine computes every solar term from 1900 to 2050 by searching for the instant the sun’s apparent longitude reaches each fifteen-degree mark, using an open-source astronomy library, and keeps the result as an absolute instant alongside its Korean clock time. Checked against the values published by the Korea Astronomy and Space Science Institute for 2026, the twelve section terms agree to within a minute, which is the almanac’s own rounding.

If you were born outside Korea, the term is converted into the clock of your birthplace before comparison, so a birth in Los Angeles is measured against ipchun in Los Angeles time. When that comparison moves your year or month pillar off the day-level answer, the chart says so — “solar-term boundary adjusted to the minute” — rather than staying silent. The one case we cannot resolve is a birth on a boundary day with no birth time; then the chart stays at the day level and is flagged as time unknown.