The magic of Cheoseo

Han Seon-hwa

The author is an honorary professor at the University of Science and Technology

Even when a heat wave seems as if it will never end, cool winds can arrive almost miraculously around Cheoseo, the traditional seasonal marker signaling the retreat of summer heat. People exhausted by tropical nights marvel at the change, calling it the “magic of Cheoseo.” Yet behind the apparent magic lies precise science.

The 24 traditional seasonal divisions are often mistaken for a lunar calendar, but they are actually based on Earth’s orbit around the sun. The vernal equinox is set at zero degrees of the sun’s ecliptic longitude, with the orbit divided into 24 intervals of 15 degrees. As Earth passes each point, roughly every 15 days, the calendar provides a remarkably precise seasonal signal.

Because Earth’s rotational axis is tilted, the angle and duration of sunlight vary according to the sun’s apparent position. At the summer solstice, the sun is high in the sky, its rays strike more directly and daylight lasts longest. The ground therefore accumulates the greatest amount of solar heat. At the winter solstice, the sun is lower, its energy is spread across a wider area and daylight is shortest.

That explains why Soseo and Daeseo, traditionally the hottest periods, come after the summer solstice, while Sohan and Daehan, the coldest periods, follow the winter solstice. The delay results from thermal inertia: Earth’s surface takes time to accumulate or release heat. Cheoseo marks the point when, after the land has absorbed the summer sun’s heat, cooling finally begins to prevail.

This system does not merely attach poetic names to changes in weather. It measures Earth’s position in its annual journey around the sun, linking astronomical motion to recurring changes in temperature, daylight and the seasons experienced on the ground.

The same principle also explains why the hottest and coldest days do not coincide with the solstices themselves. Just as a heated object remains warm after the heat source weakens, the land and atmosphere respond with a delay. By Cheoseo, the balance begins shifting decisively away from accumulated summer heat.

Climate change has recently raised concerns that the remarkable precision of these seasonal markers is weakening. Yet the “magic of Cheoseo” is also a reminder that the immense cosmic order created by Earth’s orbit and the planet’s capacity for recovery continue to operate.

Perhaps the greatest lesson of the 24 seasonal divisions is nature’s cyclic ability to regain balance while following its established course. The cool winds of Cheoseo invite awe at that vast natural cycle. But they also carry a quiet warning: Humanity must now confront climate change if it is to protect the finely balanced order on which those familiar seasonal rhythms depend.