The Vietnamese Lunar Calendar Is Actually a Lunisolar Calendar: Why the Shortened Name 'Lunar Calendar' Causes Misunderstanding

Why is it called a lunar calendar when it is actually a lunisolar calendar?

In the daily life of Vietnamese people, the phrase "lunar calendar" has become so familiar that few people question its accuracy. Tết Nguyên đán, the full moon day, the first day of the month, death anniversaries, weddings and ground-breaking ceremonies are all reckoned by the lunar calendar. Yet according to calendar researchers, this shorthand obscures an important scientific fact: the calendar that Vietnamese people use is not a purely lunar calendar, but a lunisolar calendar.

The official definition of the United States Naval Observatory also confirms this: a lunisolar calendar is a calendar that tries to stay in sync with both the solar year and the phases of the Moon. And because a solar year does not contain a whole number of lunar months, this kind of calendar is forced to adjust the length of both the year and the month, rather than simply counting lunar cycles.

This confusion does not exist only in folk life. It also appears in textbooks, causing many students to misunderstand the nature of the calendar tied to Eastern culture.

Table comparing the characteristics of a purely lunar calendar and a lunisolar calendar according to the facts in the article.

What is a purely lunar calendar?

The Moon's cycle and the unit of the month

According to a source from VietNamNet, the lunar year is reckoned by the cycle of the Moon's waxing and waning, also called Thái âm. The ancients discovered that the Moon waxes and wanes according to a rule, and on average each waxing and waning lasts 29.53 days. They took that span of time as a unit for measuring time and called it a month. A full month has 30 days, a short month has 29 days.

Because within the cycle from cold days to hot days and back again, the Moon changes from full to new more than 12 times, the ancients took 12 lunar months as one lunar year. That year has 354 or 355 days. This is the truly lunar calendar.

The origin of the lunar calendar

According to the page Nghiên cứu lịch sử cited on VietNamNet, the compilation of a calendar based on the operation of the Moon did not begin in China as many people mistakenly believe. Archaeological evidence shows that the Sumerian civilization, which developed beside the Tigris river more than 6000 years ago, in the area that later became Baghdad, the capital of Iraq, already knew how to make a calendar based on the operation of the Moon. The Roman Empire before the time of Julius Cesar also used this kind of lunar calendar.

In the book Cơ sở văn hóa Việt Nam, professor Trần Ngọc Thêm states that the lunar calendar originated in the Mesopotamian cultural region based on the movement of the Moon with a cycle of 29.53 days, with 354 days in each year.

Why is a purely lunar calendar not enough?

The gap with the four-season cycle

According to VietNamNet, by the lunar calendar, one weather cycle of changing hot and cold is 364 days, while a lunar year has only 354 to 355 days. Each year has 10 to 11 days left over, and three consecutive years leave more than a month. If only a purely lunar calendar were used, the months would drift gradually through the seasons, causing difficulties for farming and for activities tied to the weather.

What is the leap month solution?

To match the cycle of changing hot and cold weather, the ancients added one month in the third year. That year would have 13 months, the added month is called the leap month, and that year has 384 or 385 days.

The weather changes from hot to cold because the Earth rotates at a tilt around the Sun. The Earth goes around the Sun once, and the weather changes from hot to cold once. This one revolution is the basis for forming the solar calendar. Therefore, using the method of adding a leap month to reckon a calendar that matches the cycle of changing weather is precisely a combination of the lunar calendar and the solar calendar. A calendar reckoned this way is no longer a purely lunar calendar but a combination of the lunar and solar calendars.

The phrasing "add one month in the third year" above is a simplified way of speaking for easy remembering. The real rhythm of inserting leap months is tied to the Metonic cycle: 235 lunar months is approximately 19 solar years, and within those roughly 19 years there are usually 7 leap months. The Metonic cycle is a classical tool for placing leap months, but it itself is off by about two hours per cycle, so the current Vietnamese calendar does not add or subtract according to that cycle but instead calculates the new moon point and the trung khí of each year.

What do experts say about the name "lunar calendar"?

The view of calendar researcher Trần Tiến Bình

According to Báo Dân trí, ThS Trần Tiến Bình, a calendar researcher and author of the book Lịch Việt Nam thế kỷ XX-XXI, comments that what current textbooks say is clearly incomplete because it makes students misunderstand that the lunar calendar used in our country is a purely lunar calendar, following only the lunar cycle.

According to Mr. Bình, the lunar calendar in Vietnam should correctly be called a lunisolar calendar because it both follows the lunar cycle and is tied to the movement of the Earth around the Sun. The lunar calendar, called by shorthand out of habit in Vietnam, has the first day of the month called the No Moon day or the Sóc day. The Sóc day is precisely when the Earth, the Moon and the Sun lie in a straight line in that order. The Moon turns its dark half toward the Earth, so we often say "dark as the night of the 30th".

Calculating successive Sóc points gives the lunar months, which is a calendar based on the lunar cycle. But to match the weather and climate, our calendar also ties these lunar months to the 24 tiết khí such as Lập xuân, Xuân phân, Cốc vũ, Hạ chí, Tiểu mãn, Đại hàn. Each tiết khí corresponds to a certain position of the Earth around the Sun, for example Xuân phân corresponds to an angle of 0 degrees, Thanh minh corresponds to an angle of 15 degrees, Cốc vũ corresponds to an angle of 30 degrees, Kinh trập corresponds to an angle of 345 degrees.

Comparing the lunar months with the tiết khí points, specifically the Trung khí, helps us insert additional leap months. Normally a lunar year has only 12 lunar months, and a leap year has 13 lunar months. This helps overcome the difference in length between the 12 lunar months, which have only a little over 354 days, and the solar year of more than 365 days. Because of the gap of about 11 days between the lunar year and the solar year, every few years we see a leap month appear in the lunar calendar. For example, the year Ất Tỵ has a leap sixth month.

Application in education

According to Báo Dân trí, Ms. N.T.T.B, a geography teacher in Hà Nội, says that the definition of the lunar calendar in textbooks is incomplete and can cause misunderstanding about the lunar calendar being used in Vietnam as well as in many other countries. Very many Vietnamese people think that the lunar calendar used by Vietnamese or Chinese people is a calendar reckoned purely by the cycle of the Moon's movement. Meanwhile, the lunar calendar in our country is actually a lunisolar calendar.

Ms. B. comments that in the Cánh diều textbook, the application section has a question about which calendar Vietnam's Tết Nguyên đán is reckoned by. If students study only according to the general definition of the lunar calendar and the solar calendar, they will answer "lunar calendar". This is a wrong answer.

The representative of the Center for Information and Documentation, Vietnam Academy of Science and Technology, on the other hand, says that because the book only defines the solar calendar and the lunar calendar in general, the concept stated in the book is accurate and complete.

How does the Vietnamese lunisolar calendar work?

What are the Sóc day and the lunar month?

According to Báo Thanh Niên, Mr. Trần Tiến Bình says that the length of a lunar month is not fixed but changes continuously because the lunar month depends on the activity of three celestial bodies at the same time, namely the Earth, the Moon and the Sun, and not only on the Moon's cycle as people mistakenly believe.

The first day of the lunar month is the day when the Earth, the Moon and the Sun lie in a straight line in that order. The Moon turns its dark half toward the Earth, which is why our elders often say "dark as the night of the 30th". This day is also called the no moon day or the Sóc day. When calculation shows the Sóc point falling at any time of day from 0 to 24 hours, that day is the first day of the lunar month. A lunar month is the length between two Sóc days, rounded to 29 days, called a short month, or 30 days, called a full month. The length of a lunar month varies between 29.27 and 29.84 days, averaging 29.53 days.

The trung khí anchoring four months to the four seasons

The key point that keeps the lunar months from drifting through the seasons is four major trung khí anchored fixedly to four months: Xuân phân always falls in the second lunar month, Hạ chí always falls in the fifth month, Thu phân always falls in the eighth month, and Đông chí always falls in the eleventh month. Thanks to these four markers, however long or short the lunar months are, the seasons do not shift.

How a leap year is determined

Technically, to know whether a year is ordinary or leap, the calendar maker takes the Sóc point A immediately before the first Đông chí and the Sóc point B immediately before the second Đông chí. If the distance from A to B is under 365 days, that year is an ordinary year with 12 months. If the distance from A to B is over 365 days, that year is a leap year with 13 months. After the leap year has been determined, the first month after Đông chí that contains no trung khí is chosen as the leap month.

The influence of calculation error

Calculating the movement of celestial bodies is very complex, especially for the Moon because it is affected by perturbing gravitational pulls from the Sun, the Earth, other planets and the uneven shape of the Earth's sphere as well as the Moon's. Therefore, sometimes a calculation error of only a few tens of seconds leads to different short and full months.

For example, according to a publication issued in 2002 by the Purple Mountain Observatory, China, the dates 28.9.2057, 4.9.2089 and 7.8.2097 were miscalculated to the following day, which changed the length of the corresponding months. The Sóc point of 28.9.2057 fell at 23 hours 59, the Sóc point of 4.9.2089 fell at 23 hours 57, and the Sóc point of 7.8.2097 fell at 23 hours 59.

Therefore, tallying up which years have full or short twelfth months does not carry much meaning, since everything depends on accurate calculation at each specific moment.

Examples of full and short twelfth months

Mr. Bình also says that the years 2014, 2015, 2017, 2018, 2019, 2020 and 2021 had twelfth months of a full 30 days. But in 2022 the twelfth month was short at 29 days. In 2023 and 2024 it was full again. Then short twelfth months continue until 2033, when a full twelfth month appears again.

According to Lịch Vạn niên thế kỷ XX-XXI by author Trần Tiến Bình, the lunar calendar in 2024 has a 30th day of the twelfth month of Quý Mão, that is, the 30th day of Tết Giáp Thìn. However, after that, until 2032, every year the twelfth month is a short month. It is not until 2033 that there is again a 30th day of the twelfth month.

Why is the Vietnamese calendar sometimes different from the Chinese calendar?

The Sóc point and the trung khí are moments in time, not days. To know which day a moment falls on, it must be converted to the time zone of the place making the calendar. Because a calendar is a social convention rather than a purely scientific theory, which time zone is chosen is an administrative decision. NASA's astronomy book and L. E. Doggett's documents both state this clearly.

According to Báo Thanh Niên, the way the lunar calendar is calculated in Vietnam is the same as in China, both applying the same common rule. That is, the first day of the month is the Sóc day, an ordinary year has 12 months, a leap year has 13 months, the Đông chí day always falls in the eleventh month, in a leap year the month with no trung khí is the leap month, and this month is numbered the same as the month before it with the word "leap" added. If in a leap year there are two months with no trung khí, then the first month after Đông chí is considered the leap month. The calculation is based on the legal meridian, which in Vietnam is 105 degrees east longitude and in China is 120 degrees east longitude, corresponding to the seventh and eighth time zones.

Thus, the reason the Vietnamese calendar is sometimes different from the Chinese calendar is the last point mentioned above. This is based on Decision No. 121-CP of 8.8.1967 signed by the late Prime Minister Phạm Văn Đồng, stipulating that our country's official time zone is the seventh time zone and that the lunar calendar used in Vietnam is reckoned according to this official time zone.

For example, in 1968, Vietnam began using a calendar reckoned by the seventh time zone. As a result, Vietnam celebrated Tết one day before China. The reason is that 31.12.1967 corresponded to the first day of the twelfth month in both the Vietnamese and Chinese calendars, but the next Sóc point occurred at 23 hours 29 Vietnam time on 29.1.1968, so the twelfth month in the Vietnamese calendar had only 29 days, that is, a short month. At this point, by Beijing time, China was already at 0 hours 29 on 30.1, that is, a full month. Therefore, the first day of Tết Nguyên đán Mậu Thân 1968 in Vietnam was one day earlier than in China.

Other lunisolar calendars in the world

According to Báo Dân trí, Mr. Trần Tiến Bình adds that besides the East Asian lunisolar calendar being used in Vietnam, China and Korea, there are also lunisolar calendars elsewhere such as the Hebrew calendar and the Hindu calendar. The purely lunar calendar, following only the lunar cycle, is the Islamic calendar. Accordingly, the first day of the year 2025 by the Islamic calendar falls around 8 or 9 July of the solar calendar.

In our country, by habit it is still called the lunar calendar, and Tết Âm lịch. But when teaching students, it needs to be stated fully and accurately.

Tết Nguyên đán and how it is named

According to Báo VietNamNet, Tết Nguyên đán also has other names: Tết Cả, meaning the greatest festival of the year, Tết Ta to distinguish it from Tết Tây, Tết Âm lịch to distinguish it from the solar calendar, and Tết cổ truyền, today called in short form Tết.

According to professor and doctor of science Trần Ngọc Thêm, Vietnam in particular and East Asian countries in general belong to the wet-rice agricultural civilization. Because farming depends greatly on nature, the ancients divided the time of a year into 24 tiết khí to make it easier to calculate the cycle of sowing and harvesting crops. The most important of these is the opening tiết of a farming and planting cycle, that is, Tiết Nguyên đán, later mispronounced as Tết Nguyên đán.

As for the position of Tết on the calendar, Tết Nguyên đán falls on the second Sóc day after Đông chí, or the third if that year has a leap eleventh month or a leap twelfth month inserted in between. Therefore Tết always falls between late January and mid-February of the solar calendar, and never drifts into summer. Other festivals such as Tết Đoan Ngọ (the 5th day of the 5th month) and Tết Trung Thu (the full moon of the 8th month) also keep to the right season by a similar principle.

Japan is the Asian country that has abandoned Tết Nguyên đán. Ganjitsu, Japan's traditional New Year today, falls on 1 January, the first day of the solar year. Thus, this country celebrates New Year about a month earlier than its neighbouring countries. Although it does not hold Tết Nguyên đán, traditional customs and festivals are still maintained by Japanese people during the solar New Year, such as lion dances, hanging red lanterns, cooking traditional dishes and commemorative parades.

Table comparing the three kinds of calendar

The table above shows the key point: a lunisolar calendar is not "a lunar calendar plus a bit of solar calendar", but a system of its own, with complex rules for inserting months, which cannot be derived from the Gregorian calendar by subtracting a fixed number.