1. The 24 Solar Terms Define "Spring" by the Sun's Movement, Not by Temperature
The 24 Solar Terms are markers that divide the year into 24 parts based on the movement of the sun [1]. Think of it like putting 24 marks on a school calendar to break up the year. However, unlike a school schedule where people choose the dates, these marks are determined by the sun's actual position.
Four of these terms are called "Nishi-nibun" (the two equinoxes and two solstices). Another four are called "Shi-ryu" (the four starts of the seasons). Risshun (the Start of Spring) is the first of the "Shi-ryu" [1]. This is why the calendar says spring has begun in early February, even if you are wearing a heavy coat. The rule is about the sun, not the thermometer.
2. The 24 Solar Terms Began Along China's Yellow River and Were Brought to Japan
The 24 Solar Terms are said to have originated in ancient China, along the Yellow River [6]. They were later brought to Japan. However, the exact year they arrived and the path they took are not clearly known [6].
This raises a curious question: If these markers were made for the climate of the Yellow River region, do they fit an island nation like Japan, which is heavily influenced by the ocean? To understand this, we must first look at how the year was divided into 24 parts.
3. Heiki-ho: Dividing the Year into 24 Equal Periods of About 15.2 Days
The earliest method was called "Heiki-ho." It divided the length of the year by 24 [2]. This meant each period was about 15.2 days long, and the length was always the same.
This method is simple, but it has a flaw. The Earth does not orbit the sun at a constant speed. Because the speed changes, dividing the year by time causes a mismatch with the sun's actual position in the sky. Japan used this method until the Kansei Calendar [2].
4. Teiki-ho: Dividing the Year by the Sun's Position, Giving Periods of 14 to 16 Days
To fix the mismatch, Japan adopted the "Teiki-ho" method. This method uses the sun's position in the sky, measured in degrees [2]. The sun travels 360 degrees in a year. Dividing 360 by 24 gives 15 degrees per term. A new term begins the moment the sun reaches 0°, 15°, 30°, and so on.
Because the Earth moves faster when it is closer to the sun (perihelion) and slower when it is farther away (aphelion), the time between terms is not equal. The intervals vary between 14, 15, and 16 days [2]. Near early January, the intervals are shorter. Near early July, they are longer.
Japan officially used this method starting with the Tenpo Calendar, which was enforced in 1844 and used until 1872 [2][3]. Because it is based on the sun's exact position, the moment can be precise. For example, the Vernal Equinox in 2026 (when the sun reaches 0°) was March 20 at 23:46 [4]. It was just 14 minutes before midnight.
5. Japan Revised the Calendars It Received Several Times
Japan did not just accept foreign calendars. They were revised several times: the Jōkyō Calendar (1685–), the Hōryaku Calendar (1755–), the Kansei Calendar (1798–1843), and the Tenpo Calendar (1844–1872) [3].
The Jōkyō Calendar was created by Shibukawa Harumi. He used the Chinese Shoushi Calendar as a base but corrected it based on his own observations, accounting for the difference in longitude between China and Japan. This was the first calendar created by Japanese hands [3].
The Kansei Calendar, created by Takahashi Yoshitoki, incorporated Western astronomy. It is said to have used the concept of elliptical orbits for the sun and moon [3]. Through these changes, people observed the sky and made the calendar more accurate step by step.
6. Zassetsu: Japan's Extra Seasonal Markers for Farming
Alongside the 24 Solar Terms, Japan has extra seasonal markers called "Zassetsu." These were developed in Japan to help with farming [5]. Examples include Setsubun, Higan, Shobin, Hachiju-hachiya (88th Night), Nyubai (Rainy Season Start), Hange, Nijū-tōka (210th Day), and Doyo.
The counting method is specific. Hachiju-hachiya is the 88th day, counting Risshun as the first day. Nijū-tōka is the 210th day [5]. If Risshun is Day 1, then in 2026, Risshun was February 4. This means Hachiju-hachiya was May 2, and Nijū-tōka was September 1 [4].
Nyubai is defined as the day the sun reaches 80° longitude, and Hange is when it reaches 100° [4]. In 2026, Nyubai was June 11, and Hange was July 2 [4]. Doyo is the roughly 18-day period before each of the four "Shi-ryu" terms (Start of Spring, Summer, Autumn, Winter). It happens four times a year, not just in summer [5].
7. How the Solar Term Dates Are Announced Each Year and Why They Differ from the Real Seasons
The National Astronomical Observatory of Japan announces the dates for the 24 Solar Terms and Zassetsu for the next year around February. These are published as the "Reki Youkou" (Calendar Essentials) and appear in the official government gazette [4]. The dates can shift by about one day each year, so they must be recalculated annually.
Because the 24 Solar Terms were based on the Yellow River region, there is a slight mismatch with the actual seasons in Japan, which is an island nation affected by the sea [7]. The Zassetsu markers are thought to have become fixed in the Japanese calendar because they matched local farming needs and seasonal feelings [5].
It is worth noting that the "24 Solar Terms" were inscribed on UNESCO's Intangible Cultural Heritage list in 2016. However, the registration was made by China, not Japan [1].
8. Try It Yourself: Marking the Sun's Light in Your Room to See Its Path
You can see the sun's movement with your own eyes. Find a spot where sunlight hits the floor or wall from your window. At the same time every day, mark the edge of the sunlight with tape or place a small object there. Write down the date. After about a week, check the position again at the same time. Compare how far and in which direction the light has moved.
This simple experiment shows how the sun's position changes. The 24 Solar Terms are defined by the exact moment the sun reaches a specific angle. This activity lets you see that change in your home.
Safety note: Never look directly at the sun. Use indirect sunlight on a wall or floor. If you want to learn more, the National Astronomical Observatory's "Koyomi Wiki" has explanations in Japanese about how these terms are set and what Zassetsu are [2][5]. You can also look for books about calendars in your local library.