The 24 Solar Terms

For farmers in the past, weather and seasons were a matter of survival. They had to know exactly when to sow seeds and when to harvest, or the whole year's crop could fail.

The lunar calendar counts days by the moon's phases. You can read the date just by looking up at the night sky, so it was very handy for daily life. But a purely lunar year is only about 354 days long (a lunar cycle is about 29.5 days). That makes it drift about 11 days earlier than the real seasons every year. Farm by it long enough, and you might end up planting seeds in the middle of winter.

So people divided the sun's path through the year into 24 stations, like stops on a train line. At first, they split the year into 24 equal chunks of days. In Korea, after the Sihyeon calendar arrived in the late Joseon period (1653), the system became more precise: the sun's path (360 degrees) was divided into 15-degree steps, as it is today. From Ipchun, the start of spring, through Useu, when ice melts, to Gyeongchip, when frogs wake up, these marks worked like traffic lights telling farmers when to sow and when to reap.

The calendar names, like Ipchun (start of spring), Haji (summer solstice), and Dongji (winter solstice), are written in Chinese characters. So many people assume they are lunar dates. But look closely at when they fall. Every year, they land on remarkably steady dates in the solar (Gregorian) calendar: around the 4th to 8th and the 18th to 24th of each month.

That is because the terms have nothing to do with the moon's phases. They are set only by Earth's position in its orbit around the sun. Each time Earth moves another 15 degrees, a new term begins. That is why they line up almost perfectly with the calendar we use today.

This is also the secret behind how people who used the lunar calendar could still keep their farming schedule right through all four seasons. The 24 solar terms only look like part of an old lunar calendar. Underneath, they are a precise solar calendar based on the positions of Earth and the sun.

To be a bit more precise, the 24 solar terms were made about 2,000 years ago by observing the climate along the Yellow River in ancient China. So they don't match the real weather and seasonal feel of the Korean Peninsula perfectly, since its location and terrain are different.

A classic example: Daehan means "greatest cold," and Sohan, which comes just before it, means "lesser cold." Yet in Korea, the days around Sohan often feel much colder than those around Daehan. Koreans even have funny sayings about it, like "Daehan went to visit Sohan's house and froze to death" and "Sohan's ice melts in Daehan." That climate gap is why.

Today, climate change is shifting the weather quickly, so term names and real weather don't always match. Still, the 24 solar terms remain a handy wisdom, a clock that shows the big flow and cycle of the seasons: how nature wakes up, ripens, and falls asleep over a year.