The 24 Solar Terms — How Korea Divides the Sun's Path Into Twenty-Four

What a Solar Term Is — Ticks on the Ecliptic

Seen from Earth, the sun appears to travel a full circle through the sky over the course of a year. That apparent path is called the ecliptic. The 24 solar terms simply cut that 360-degree circle into twenty-four slices of 15 degrees each and name the moment the sun crosses each tick. The spring equinox sits at ecliptic longitude 0; 15 degrees later comes Cheongmyeong, 30 degrees Gogu, and 90 degrees the summer solstice.

Because the sun is the reference, the terms line up exactly with the seasons. Haji is always the longest day in the northern hemisphere and Dongji always the longest night. A purely lunar year, by contrast, runs about 354 days and drifts more than ten days away from the seasons annually. Farmers needed to know when to sow and when to harvest, and the moon alone could not tell them — hence a separate scale that reads the sun.

Fifteen degrees works out to roughly fifteen days between terms. Earth's orbit is an ellipse rather than a circle, though, so the sun crosses the ecliptic at slightly different speeds through the year. In winter, when Earth is closer to the sun and moving faster, the gap narrows toward fourteen days; in summer it stretches toward sixteen. This is also why the calendar date of each term wobbles by about a day from year to year.

Jeol and Gi — A Two-Layer Structure

The phrase "24 solar terms" actually covers two interleaved sets of twelve. Ipchun, Gyeongchip, Cheongmyeong, Ipha, Mangjong, Soseo, Ipchu, Baengno, Hallo, Ipdong, Daeseol, and Sohan are the twelve jeol ("nodes"). Usu, Chunbun, Gogu, Soman, Haji, Daeseo, Cheoseo, Chubun, Sanggang, Soseol, Dongji, and Daehan are the twelve gi ("mid-terms"). The two alternate around the year.

The distinction is functional, not decorative. A jeol marks the threshold of a month: in Korean saju, the month pillar changes exactly at a jeol, so the tiger month begins at Ipchun and the rabbit month at Gyeongchip. A gi, by contrast, sits at the heart of a month and serves as its name tag in the lunar calendar.

Put simply, jeol are the fences between months and gi are the labels that confirm which month you are in. As we will see, the entire rule for inserting a leap month hinges on whether a lunar month contains a gi.

The Full List of Twenty-Four

The table below lists all twenty-four terms in order. Gregorian dates can shift by about a day depending on the year, so approximate dates are given. Most names describe an observed natural phenomenon, so reading down the list is itself a sketch of how a year unfolds in Korea.

Note that a few names run ahead of lived experience. Ipchu ("autumn begins") falls in early August, the peak of summer heat, and Ipchun ("spring begins") falls in early February, often the coldest stretch of the year. A term name does not announce that a season has arrived — it marks the moment its energy begins to turn.

Why the Traditional Year Begins at Ipchun

People born in January or early February are often surprised to be assigned the previous year's zodiac sign in a saju reading. The reason is that saju uses neither the Gregorian nor the lunar calendar, but the solar-term calendar. In that system the year begins at Ipchun, and the boundaries between the twelve months are the twelve jeol described above.

Ipchun was chosen because Korean fortune-telling grew out of agricultural observation of the seasons. After the winter solstice the sun begins climbing again, and Ipchun is the point where that returning energy first becomes visible on the ground. In other words, the year starts where a new cycle actually begins. January 1st carries no such astronomical or seasonal meaning — it is an administrative convention.

As a result, someone born on February 3rd and someone born on February 5th can have entirely different year pillars. Ipchun is also reckoned to the minute, not just the day: if Ipchun falls at 11 am in a given year, a baby born that same morning still belongs to the previous year. This is why careful readings ask for the exact time of birth.

Solar Terms in Farming and Seasonal Custom

For centuries the terms were the farming calendar itself. Seedbeds were prepared around Cheongmyeong and Gogu, rice transplanting finished by Mangjong, and weeding wrapped up once Cheoseo had passed and the grain began to ripen. Proverbs such as "after Cheoseo, even the mosquito's mouth goes crooked" compress generations of accumulated observation into a single line.

Many customs attach to specific terms. On Dongji, families cooked and shared red bean porridge — red was believed to repel misfortune, and the longest night was a fitting moment to welcome a returning sun. At Ipchun, households pasted couplets on gates and pillars; the most common reads "Ipchun daegil, geonyang dagyeong," a wish for great fortune and abundant blessings with the returning light.

Even the three hottest days of summer, the sambok, are set by the terms. Chobok falls on the third gyeong day after Haji, Jungbok on the fourth, and Malbok on the first gyeong day after Ipchu — which is why the dates move each year and why the gap before Malbok sometimes stretches to twenty days. Hansik, likewise, falls 105 days after Dongji, landing near Cheongmyeong.

Solar Terms and the Lunar Calendar — Where Leap Months Come From

A lunar month follows the cycle of the moon's phases, about 29.53 days, so twelve of them total roughly 354 days — about eleven days short of the sun's 365.24-day circuit. Left alone, the calendar would slip a full month within three years and turn the seasons upside down within a couple of decades. The fix is to insert an extra month every few years: a leap month.

The question is which month to double. Traditional calendar-making answers it with the solar terms. Each of the twelve gi acts as a name tag for a lunar month, and because lunar months are short, one occasionally slips through without containing any gi at all. That month becomes the leap month — a rule known as "no mid-term, insert a leap." It borrows the preceding month's name, giving names like leap fourth month or leap sixth month.

This scheme adds seven leap months every nineteen years, keeping lunar dates from drifting away from the seasons over the long run. What Koreans call the lunar calendar is therefore not purely lunar at all: it is a lunisolar calendar corrected by the sun's own scale. Once you understand the terms, the lunar calendar stops looking arbitrary.

Reading the Solar Terms Today

You do not need a rice paddy for the terms to be useful. The late heat still tends to break around Cheoseo, mornings still turn crisp after Baengno, and the first frost still shows up near Sanggang. Knowing the terms turns the change of seasons from a vague feeling into a predictable rhythm — handy for deciding when to swap out a wardrobe or when to plan a trip.

It is worth being honest, though, that climate change has widened the gap between term names and actual weather. A solar term corresponds precisely to an astronomical fact — the sun's position — but guarantees nothing about that day's temperature or rainfall. Treat the terms as a seasonal coordinate system, and check the forecast for the forecast.

Unse Hanjan computes solar term dates itself rather than calling an external service. We derive them from an astronomical formula for the sun's ecliptic longitude, then cross-check and correct against the values published by the Korea Astronomy and Space Science Institute. The same computed results drive our auspicious-day calendar and the year boundary used in the New Year fortune, so the terms are a working reference here, not decoration.

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