Why does a lunar almanac contain a solar component?
Open a Vietnamese perpetual calendar and you see two layers of information overlapping: on one side, lunar dates with the first, the fifteenth, and the thirtieth; on the other, solar terms such as Start of Spring, Clear and Bright, and Winter Solstice. Many people assume the solar terms are also part of the lunar calendar. In fact, this is one of the most common misunderstandings about the traditional Eastern calendar.
According to researcher Dang Vu Tuan Son, Chairman of the Vietnam Astronomy and Cosmology Association, the lunar calendar is based on the lunar cycle: every 12 lunar months make one year. But weather cycles do not depend on the Moon; they depend mainly on Earth's orbit around the Sun, because the Sun is the main source of heat for Earth. Therefore, the solar terms do not belong to the lunar calendar.
Yet saying the solar terms belong to the solar calendar is also inaccurate. Today's solar calendar, properly called the Gregorian calendar, was issued by Pope Gregory XIII at the end of the 16th century, revised from Julius Caesar's Julian calendar. The solar terms were established by Eastern convention, with their own history and frame of reference. The only meeting point is that both systems rest on the same astronomical motion: Earth revolving around the Sun.
In other words, the traditional lunar almanac is actually a lunisolar calendar. It uses lunar months to count days, but also records solar terms to reflect the weather. These two layers are not of the same nature, yet they are bound into one almanac for practical use.
The astronomy behind the 24 solar terms
According to Earth science and astronomy, Earth's orbit is an ellipse, so its speed along its orbit around the Sun is not a fixed constant. When Earth is closer to the Sun, it moves faster; when farther, it slows down. As a result, the time gap between solar terms is not fixed, ranging from about 14 to 16 days.
A year has 4 seasons, each with 6 solar terms, 24 in total. Each term is a period of 15 or 16 days describing a certain weather phase. The starting point of a term is determined by the Sun's position on the ecliptic, the apparent path of the Sun across the sky.
Because it is based on Earth's position on its orbit, the solar terms nearly match the solar calendar. This is why, to know when a given term begins each year, the most convenient reference is the solar calendar, not the lunar one.
Distribution of dates across the year
According to feng shui expert Nguyen Song Ha, in the first 6 months of the solar year, solar terms usually fall on the 5th and mid-terms on the 21st. In the last 6 months, solar terms fall on the 8th and mid-terms on the 23rd. These markers may vary by about 1 to 2 days. For example, Start of Spring usually falls on February 3 to 5 of the solar calendar.
This distribution reflects the stable periodicity of the solar terms under the solar calendar, quite unlike the lunar month, which shifts every year.
The 24 solar terms and the meaning of each group
Each solar term's name carries its own meaning, reflecting a natural phenomenon or farming activity. They can be grouped as follows:
- The Four Starts group includes Start of Spring, Start of Summer, Start of Autumn, and Start of Winter, reflecting when the four seasons begin.
- The equinox and solstice group includes Spring Equinox, Autumn Equinox, Summer Solstice, and Winter Solstice, marking special astronomical points.
- The humidity group includes Minor Heat, Major Heat, End of Heat, Minor Cold, and Major Cold, reflecting changes in humidity and temperature.
- The weather group includes Rain Water, Grain Rain, White Dew, Cold Dew, Frost Descent, Minor Snow, and Major Snow, reflecting specific weather phenomena.
- The seasonal group includes Awakening of Insects, Clear and Bright, Grain Full, and Grain in Ear, reflecting seasonal phenomena of animals and plants.
Specifically, Awakening of Insects is when insects breed heavily. Grain in Ear is when grains come into ear. Grain Full suggests the level of accumulated water or grain. Clear and Bright is tied to clear skies and green vegetation.
Four seasons and six terms each
- Spring includes Start of Spring, Rain Water, Awakening of Insects, Spring Equinox, Clear and Bright, and Grain Rain.
- Summer includes Start of Summer, Grain Full, Grain in Ear, Summer Solstice, Minor Heat, and Major Heat.
- Autumn includes Start of Autumn, End of Heat, White Dew, Autumn Equinox, Cold Dew, and Frost Descent.
- Winter includes Start of Winter, Minor Snow, Major Snow, Winter Solstice, Minor Cold, and Major Cold.
Why the solar terms are tightly bound to farming
This is the core reason the solar terms have survived for thousands of years. Traditional agriculture depends on weather, and weather depends on Earth's position on its orbit around the Sun. The lunar month, by contrast, reflects only the lunar cycle, not the weather cycle.
Researcher Dang Vu Tuan Son points out that Earth's orbital cycle is about 10 days longer than the total length of 12 lunar months. Because of this gap, a purely lunar calendar cannot reflect the weather cycle. Ancient Eastern people added a leap month, a 13th month, to make up the difference. Usually one in every 3 years is a leap year, so the lunar calendar catches up with the weather cycle.
Even so, even with a leap month, the lunar month is still not an accurate unit for measuring weather. For example, the saying "July brings the drizzle" is relative folk experience. The seventh lunar month is not an exact period; it varies by 10 to 20 days each year when comparing July of two consecutive years. In northern Vietnam and many regions, autumn often has light but prolonged rains called drizzle, but there is no basis to claim this phenomenon occurs only in the seventh lunar month.
The solar terms, by contrast, do not depend on the lunar calendar; they depend only on the weather cycle. This is why the solar terms became a far more accurate guide for farming. Ancient farmers relied on the solar terms to decide when to sow, harvest, raise livestock, and manage irrigation.
Specific examples of farming application
- Start of Spring marks the beginning of spring and of a new year by the solar terms. This is when heaven and earth shift, suitable for preparing the spring crop.
- Grain Rain suggests a time of abundant rain, favorable for the winter-spring rice to grow.
- Grain in Ear is when grains come into ear, requiring attention to water and pests.
- Frost Descent signals the onset of frost, requiring harvest before the cold.
- Winter Solstice is the midpoint of winter, the shortest day in the Northern Hemisphere, tied to keeping warm and preserving produce.
Equinoxes and solstices: special astronomical points
Among the 24 solar terms, the four markers Spring Equinox, Autumn Equinox, Summer Solstice, and Winter Solstice hold special positions. According to researcher Dang Vu Tuan Son, these are four very basic time markers, learned by students very early. Though often called "days", they are in fact not one day but a solar term lasting up to 15 days. What we call the Spring Equinox day today is only the starting day of the Spring Equinox term.
The mechanism of the equinox
In the solar calendar, strictly speaking, the Spring Equinox is a point, not a day. The Spring Equinox point is one of the two intersections of the ecliptic and the celestial equator. Earth's equator and its orbit are two planes offset by about 23.5 degrees, so there are two intersection points in the sky.
When the Sun passes through the intersection heading northward, that is the Spring Equinox point. When it heads southward, that is the Autumn Equinox, for the Northern Hemisphere, and the reverse for the Southern Hemisphere. When the Sun crosses the celestial equator, that is, lies exactly on Earth's equatorial plane, both hemispheres receive equal illumination. Thus, on the days near this point, day and night are nearly equal in length.
The English word "equinox" has Latin roots in aequinoctium, from aequus meaning equal and noctis meaning night, precisely to denote this equal day and night.
The mechanism of the solstice
The two solstice days are collectively called solstice, from Latin sol meaning Sun and sistere meaning to stand still. The Summer Solstice, for the Northern Hemisphere, is when the Sun's apparent path reaches its highest northern declination and pauses before gradually moving south. Conversely, the Winter Solstice is when the Sun reaches its highest southern declination and turns back.
Why Eastern equinoxes and solstices match Western ones
Many ask whether, in ancient times, there was any cultural exchange leading to the coincidence of equinox and solstice dates between the two systems. The answer is no. The conventions for equinoxes and solstices in the two systems are very different. We used existing names to turn them into Vietnamese terms when translating the names of the equinox and solstice points that Western astronomers observed and put into the calendar.
The coincidence is simply that the equinox and solstice dates in the solar calendar match the starting dates of those terms in the solar term system. This coincidence is easy to understand: the Sun crossing the equator is a special moment that astronomers in both ancient cultures recorded. In other words, both cultures observed the Sun reaching the Spring Equinox point and set conventions for it and applied them to the calendar.
Interesting phenomena: years without Start of Spring, years with two
One of the most visible consequences of the relationship between solar terms and the lunar calendar is the phenomenon of a lunar year without the Start of Spring term, or with two.
It is recorded that the lunar year Giap Thin 2024 had no leap month, totaling 354 days and only 23 solar terms. Meanwhile, the solar year 2024 was a leap year with 366 days. The Start of Spring term in 2024 fell on February 4, corresponding to the 25th day of the twelfth lunar month of Quy Mao. In other words, the lunar year Quy Mao 2023 had two Start of Spring days, one on the 14th of the first lunar month and one on the 25th of the twelfth lunar month, leading to the absence of the Start of Spring term in the year Giap Thin.
This phenomenon occurs because ancient calendar makers added 7 leap months to the lunar calendar every 19 years to adapt to weather changes. In 19 years there will be 7 years without the Start of Spring term, 7 years with 2 Start of Spring days, and 5 years with one. This is the result of the necessary alignment between the solar and lunar calendars.
The case of Tet Nguyen Dan 2026
A recent example is Tet Nguyen Dan Binh Ngo 2026. In 2026, the Start of Spring term begins on February 4 of the solar calendar, that is, the 17th of the twelfth lunar month of At Ty, and lasts until February 17, exactly the first day of Tet Nguyen Dan Binh Ngo. The very next day, February 18, the second day of Tet, the Rain Water term follows and lasts until March 4, the 16th of the first lunar month. Following Rain Water is the Awakening of Insects term, beginning March 5.
The reason is that the past At Ty year was quite special, as Vietnamese people welcomed the Start of Spring term twice in the year. The At Ty year began with the Start of Spring term on February 3, 2025 of the solar calendar, the 6th day of Tet, and the second time on February 4, 2026 of the solar calendar, the 17th of the twelfth lunar month. The At Ty year was a leap year, lasting up to 384 days, from January 29, 2025 to February 16, 2026, 19 days longer than the solar calendar. The reason is that Vietnamese people welcomed the sixth lunar month twice: the first sixth month had 30 days from June 25 to July 24 of the solar calendar, and the leap sixth month lasted 29 days from July 25 to August 22 of the solar calendar.
Comparing views: which system do the solar terms belong to?
Three common views can be summarized:
- Common but wrong view: The solar terms belong to the lunar calendar. This view stems from the solar terms being printed together in the lunar almanac, but it is incorrect in astronomical nature.
- Also inaccurate view: The solar terms belong to the solar calendar. This view is right that the solar terms nearly match the solar calendar, but wrong in attributing them to a calendar system issued by the West.
- Accurate view: The solar terms are a convention system of the Eastern people, based on Earth's motion around the Sun, so they nearly match the solar calendar but belong to neither the solar nor the lunar calendar.
Today, the 24 solar terms calendar is little used in modern life because it is not really necessary for most work. The names of equinoxes and solstices are mainly used to name the corresponding days in the solar calendar. Yet in agriculture, especially traditional and seasonal farming, the solar terms still hold an important reference role.
It should also be noted that today, besides cyclical changes across years according to the solar terms, the weather of each year is also heavily influenced by climate change. Therefore, the solar terms should be seen as a reference frame with long-term statistical value, not an exact forecast for each specific year.
Conclusion
The twenty-four solar terms are one of the notable astronomical observational achievements of the ancient Eastern people. The system belongs to neither the lunar nor the solar calendar, but because both rest on Earth's motion around the Sun, it nearly matches the solar calendar. It is this feature that makes the solar terms a far more accurate guide for farming than the purely lunar month.
Understanding the true nature of the solar terms helps readers of the perpetual calendar avoid confusing the two layers of information in one almanac: one layer is the lunar cycle, one is the weather cycle. At the same time, it also helps distinguish clearly between the long-term reference value of the solar terms and the short-term variability of actual weather in the context of climate change.
