TL;DR: A lunar calendar has two distinct layers. The first covers lunar dates, leap months and solar terms, which are astronomical calculations verifiable against printed calendars or independent tables. The second covers auspicious days, lucky and unlucky stars and favourable hours, which are conventions of day-selection manuals and are not derived from the actual positions of celestial bodies.
In a lunar calendar, which part is science and which part is convention?
A common lunar calendar in Vietnam is not a single unified block. It is two layers stacked on top of each other. The lower layer is the result of astronomical calculation, covering lunar dates, leap months and solar terms, and it can be verified by comparing several independent calendar sources. The upper layer is a day-selection convention, covering auspicious days, lucky and unlucky stars and favourable hours, set out and handed down by day-selection manuals, not derived from the actual positions of the Sun, the Moon or the planets.
The blurring between these two layers is why many people do not know how far to trust a calendar. The short answer: trust the calculated layer, and understand the conventional layer as convention. The calculated layer is science in the narrowest sense, meaning it can be verified. The conventional layer is culture, not science, but it is not therefore meaningless to its users.
How do you measure the gnomon's shadow to fix the solar terms?
Solar terms are the backbone of the calculated layer. To know how many solar terms a year has and which day each falls on, the old calendar-makers had to determine when the Sun passed certain points on the ecliptic. The earliest and most direct method was measuring the shadow of a gnomon.
Christopher Cullen, in Astronomy and Mathematics in Ancient China: The Zhou bi suan jing (Cambridge University Press, 1996), analyses the Zhou bi suan jing, an ancient Chinese astronomical mathematical text, and shows how people in antiquity used the gnomon's shadow to derive points in the year, including the basis for fixing the solar terms.
Cullen also shows that the Zhou bi suan jing tied shadow measurement to a cosmological model in which the Earth is flat and the Sun moves on certain circles. That model is not correct by modern astronomy, but the measuring method still produced usable numbers. This is the point to note: a technique can be correct within its range of application even when the theoretical model behind it has been replaced.
Cullen is a historian of Chinese science, and this book belongs to the body of academic work referenced by researchers. This is level A in Nhật Nguyệt's classification: consensus of researchers, stated plainly, with the author's name and year.
Why was calendar-making a state affair?
In Chinese history, the calendar was not merely a civil tool. It was a political tool. The state issued the calendar, and issuing a correct calendar was a way of asserting authority. When the calendar was wrong, the court lost credibility, so calendars were made, tested and replaced as needed.
Christopher Cullen, in Heavenly Numbers: Astronomy and Authority in Early Imperial China (Oxford University Press, 2018), sets out that in early imperial China calendrical astronomy was bound tightly to state power, and calendars were reformed across many dynasties.
Jean-Claude Martzloff, in Astronomy and Calendars: The Other Chinese Mathematics, 104 BC to AD 1644 (Springer, 2016), surveys the mathematics used for the Chinese calendar from 104 BC to AD 1644, including the calculation of solar terms and the placing of leap months.
The two books complement each other. Cullen stresses the relation between calendar and power, Martzloff stresses the mathematical technique. Both show that the calculated layer of the calendar is the product of a long computational tradition, not of mere belief.
What is a leap month and why is one needed?
A leap month is a direct consequence of combining a lunar calendar with the solar year. A lunar month is about 29.5 days, twelve lunar months are about 354 days, roughly 11 days shorter than the tropical year. Without leap months, Tet would drift through the seasons. To keep Tet within the winter and spring, calendar-makers had to insert leap months by a rule based on solar terms.
Martzloff sets out those rules in Astronomy and Calendars. This is the verifiable part: take a year with a leap month, compare it with a table of solar terms, and the rule is applied consistently. There is no element of belief in this step.
How did Jesuit missionaries bring European astronomy to Vietnam?
The Vietnamese calendar was not shaped only by the Chinese tradition. From the 16th to the 18th century, Jesuit missionaries came from Europe to East Asia, bringing Western astronomy and mathematics. They took part in calendar calculation in China and in Đàng Ngoài and Đàng Trong.
Anh Thuan Truong, in the article "Processes of Receiving Western Astronomy in China and Vietnam" (Vestnik of Saint Petersburg University. History 65(2): 469-490, 2020), sets out the process by which European astronomy was received in China and Vietnam during this period.
The article shows that the reception was not simple copying. There was selection and adjustment to local conditions. This is a part of the history of science in Vietnam rarely mentioned in popular books but recognised by researchers.
Hoàng Xuân Hãn's research on the Vietnamese calendar
An important document on the Vietnamese calendar is the collected works of Hoàng Xuân Hãn. Care is needed, however, when citing it.
The collected works La Sơn Yên Hồ Hoàng Xuân Hãn, edited by Hoàng Xuân Hãn and Hữu Ngọc (Education Publishing House, Hanoi, 1998), runs to three volumes and includes studies of the Vietnamese calendar.
What are auspicious days, lucky stars and favourable hours?
Moving to the upper layer, the story is quite different. Auspicious days, inauspicious days, lucky and unlucky stars and favourable hours are not calculated from the actual positions of the Sun or the Moon. They are set out by day-selection manuals, based on systems of symbols such as the sexagenary cycle, the twenty-eight lunar mansions, the nine stars and the eight trigrams.
Richard J. Smith, in Chinese Almanacs (Oxford University Press, 1992), surveys the structure of the good-day and bad-day entries in Chinese almanacs and shows that this is an organised system of convention, with specific rules about which day suits which activity.
The important point is this: the system has an internal logic, but that logic is not astronomical logic. It is the logic of an interpretive tradition. This does not make it meaningless. It only means something different: cultural meaning, ritual meaning, psychological meaning, not physical meaning.
Cửu Diệu and the rite of offering to the stars to avert misfortune
A concrete example of the conventional layer is the Cửu Diệu system, nine stars said to govern one's fate according to lunar age. This system is used in the custom of offering to the stars to avert misfortune.
Lê Đức Hạnh, in the article "Cung sao giai han" (Religious Studies, no. 12, 2008), cites Hoàng Tâm Liên's explanation that the full moon of the first month is also called the Thượng Nguyên festival, and that according to some Chinese books such as Đường Thư Lịch Chí, volume 18, there are nine stars shining in the sky, namely Nhật Diệu, Nguyệt Diệu, Hỏa Diệu, Thủy Diệu, Mộc Diệu, Kim Diệu, Thổ Diệu, La Hầu and Kế Đô.
The article also notes that some books speak of seven stars, some add the two stars La Hầu and Kế Đô to make nine, and some add Thái Bạch as well to make ten. The differences between books show that this is an oral convention adjusted over time, not a fixed astronomical table.
In folk belief, each lunar age is held to be influenced by a star, with good stars in some years and bad stars in others, and La Hầu and Kế Đô are bad stars, hence the custom of offering to the stars to avert misfortune at the start of the year.
This belief can affect behaviour: people avoid major undertakings in a year with a bad star, or make offerings for peace of mind. If, for peace of mind, people are more careful in their work, the outcome may improve. That is a real effect produced by belief, not evidence that the star system correctly predicts destiny. It must be put exactly that way: belief changes behaviour, behaviour changes outcomes, but it does not follow that the system has an astronomical basis.
What does the table comparing the two calendar layers show?
The table below summarises the differences between the two layers, for quick reference.
| Item | Astronomical calculation layer | Day-selection convention layer |
|---|---|---|
| Components | Lunar dates, leap months, solar terms | Auspicious days, lucky and unlucky stars, favourable hours |
| How determined | Calculated from the positions of the Sun and Moon | Set out by day-selection manuals, based on the sexagenary cycle, the twenty-eight lunar mansions, the nine stars |
| Verification | Can be checked against printed calendars and independent tables | Cannot be verified by astronomy, only compared between books |
| Origin | Tradition of gnomon shadow measurement, state calendar reform | Tradition of day selection, with its own internal logic |
| Nature | Science in the sense of being verifiable | Culture, ritual, psychology |
Note: the "astronomical calculation layer" column refers to the core of the calendar, not the claim that every number in a calendar is correct. A printed calendar can be wrong through printing errors or through different rules used in different regions. But such errors can still be detected by comparison, exactly as befits the product of calculation.
Why does distinguishing the two layers matter?
Distinguishing the two layers helps readers trust the right place. When a calendar says today is such-and-such a lunar date, with such-and-such a solar term, and the leap month falls here, we can verify it. When that calendar says today is an auspicious day, good for weddings, we are reading a convention. Both pieces of information can be useful, but they carry different degrees of certainty.
In other words, there is no need to deny the conventional layer in order to defend the calculated layer. Nor is there any need to credit the conventional layer with a scientific basis it does not have. The two layers coexist in one calendar, and a clear-headed reader is one who knows which layer they are reading.
Does the lunar calendar have a real scientific core?
The Vietnamese lunar calendar has a real scientific core: the calculation of lunar dates, leap months and solar terms, based on a long astronomical tradition and verifiable. The day-selection part is a different conventional layer, with its own internal logic and cultural value, but not the result of calculation from the sky. Stating this boundary clearly does not destroy the calendar's value. It only helps readers know which part is trustworthy in the scientific sense, and which part should be understood as cultural belief.
To learn more about the name of this kind of calendar, see The Vietnamese lunar calendar is actually a lunisolar calendar: why the shorthand "lunar calendar" causes misunderstanding. To learn more about the times when the Vietnamese and Chinese calendars diverge by a day, see Why the Vietnamese lunar calendar sometimes differs from the Chinese calendar by one day, and in some years the two countries celebrate Tet on different days.
