Why is it called the lunar calendar when it is actually a lunisolar calendar?
In Vietnamese life, the phrase lunar calendar has become so familiar that few question its accuracy. Tet Nguyen Dan, the full moon day, the first day of the month, death anniversaries, weddings, and ground-breaking ceremonies are all calculated by the lunar calendar. However, according to calendar researchers, this shortened name hides an important scientific fact: the calendar Vietnamese people use is not a purely lunar calendar, but a lunisolar calendar.
This confusion exists not only in folk life. It also appears in textbooks, causing many students to misunderstand the nature of the calendar tied to Eastern culture.
What is a purely lunar calendar?
The Moon's cycle and the unit of the month
According to VietNamNet, the lunar year is calculated by the waxing and waning cycle of the Moon, also called the lunar body. Ancient people discovered that the Moon waxes and wanes according to a rule, with each cycle lasting an average of 29.53 days. They took that period as a unit of time and called it a month. A full month has 30 days, and a deficient month has 29 days.
Because in the cycle from cold days to hot days and back, the Moon changes its waxing and waning more than 12 times, ancient people took 12 lunar months as one lunar year. That year has 354 or 355 days. This is the truly lunar calendar.
Origin of the lunar calendar
According to the Historical Research page cited on VietNamNet, compiling a calendar based on the Moon's operation did not begin in China as many mistakenly believe. Archaeological evidence shows that Sumerian civilization, which developed beside the Tigris River more than 6000 years ago in the area later known as Baghdad, the capital of Iraq, knew how to make a calendar based on the Moon's operation. The Roman Empire before Julius Caesar also used this type of lunar calendar.
In the book Foundations of Vietnamese Culture, Professor Tran Ngoc Them states that the lunar calendar originated in the Mesopotamian cultural region based on the Moon's movement with a cycle of 29.53 days, with each year having 354 days.
Why is a purely lunar calendar not enough?
Difference from the four-season cycle
According to VietNamNet, by the lunar calendar, one weather cycle of hot and cold change is 364 days, while a lunar year has only 354 to 355 days. Each year has 10 to 11 extra days, and three consecutive years have more than one extra month. If only a purely lunar calendar were used, the months would gradually drift through the seasons, causing difficulties for farming and living according to the weather.
The leap month solution
To match the cycle of hot and cold weather changes, ancient people added one month to the third year. That year would have 13 months, the added month is called a leap month, and that year has 384 or 385 days.
Hot and cold weather changes are caused by the Earth revolving around the Sun at a tilt. As the Earth completes one orbit around the Sun, the weather changes from hot to cold once. This one orbit is the basis for forming the solar calendar. Therefore, using leap months to calculate a calendar that matches the weather cycle is a combination of the lunar and solar calendars. Such a calculation is no longer a purely lunar calendar but a combination of the lunar and solar calendars.
Experts' views on the name lunar calendar
Opinion from calendar researcher Tran Tien Binh
According to Dan Tri, MSc Tran Tien Binh, calendar researcher and author of the book Vietnamese Calendar of the 20th-21st Centuries, says that writing as current textbooks do is clearly incomplete because it makes students misunderstand that the lunar calendar used in our country is a purely lunar calendar, following only the moon's phases.
According to Mr. Binh, the lunar calendar in Vietnam must accurately be called a lunisolar calendar because it follows both the moon's phases and the Earth's movement around the Sun. The lunar calendar, shortened by habit in Vietnam, has the first day of the month called the No-Moon day or New Moon day. The New Moon day is when the Earth, Moon, and Sun are in a straight line in that order. The Moon turns its dark half toward the Earth, so our elders often say dark as the night of the 30th. Calculating consecutive New Moon points gives lunar months, which is a calendar based on the moon's phases. But to match the weather and climate, our calendar also links these lunar months with the 24 solar terms such as Start of Spring, Spring Equinox, Grain Rain, Summer Solstice, Grain Full, and Great Cold. Each solar term corresponds to a certain position of the Earth around the Sun, for example Spring Equinox corresponds to 0 degrees, Clear and Bright corresponds to 15 degrees, Grain Rain corresponds to 30 degrees, and Awakening of Insects corresponds to 345 degrees.
When comparing lunar months with solar term points, specifically the mid-terms, it helps us insert 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 12 lunar months, which have only more than 354 days, and the solar year, which is longer than 365 days. Because of the difference of about 11 days between the lunar and solar years, every few years a leap month appears in the lunar calendar. For example, the year of the Snake has a leap sixth month.
Application in education
According to Dan Tri, Ms. N.T.T.B, a geography teacher in Hanoi, says that the definition of the lunar calendar in textbooks is incomplete and can cause misunderstanding about the lunar calendar used in Vietnam and many other countries. Many Vietnamese people think that the lunar calendar used by Vietnamese or Chinese people is a calendar calculated purely by the Moon's movement cycle. Meanwhile, the lunar calendar in our country is actually a lunisolar calendar.
Ms. B. says that in the Canh Dieu textbook, the application section asks what calendar Vietnamese Tet Nguyen Dan is calculated by. If students only learn the general definition of lunar and solar calendars, they will answer lunar calendar. This is a wrong answer.
The representative of the Information and Documentation Center, Vietnam Academy of Science and Technology, says that because the book only defines the solar and lunar calendars in general, the concept stated in the book is accurate and complete.
Mechanism of the Vietnamese lunisolar calendar
New Moon day and lunar months
According to Thanh Nien, Mr. Tran Tien Binh says that the length of a lunar month is not fixed but changes continuously because the lunar month depends on the simultaneous activity of three celestial bodies, including the Earth, Moon, and Sun, not only on the Moon's cycle as people mistakenly believe.
The first day of the lunar month is the day when the Earth, Moon, and Sun are in a straight line in that order. The Moon turns its dark half toward the Earth, so our elders often say dark as the night of the 30th. This day is also called the no-moon day or New Moon day. When the New Moon point falls 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 New Moon days, rounded to 29 days, called a deficient month, or 30 days, called a full month. The length of a lunar month varies from 29.27 to 29.84 days, with an average of 29.53 days.
Influence of calculation errors
Calculating the movement of celestial bodies is very complex, especially for the Moon because it is affected by disturbances from the gravitational pull of the Sun, Earth, other planets, and the uneven shapes of the Earth and Moon. Therefore, sometimes a calculation error of only a few tens of seconds leads to different deficient and full months.
For example, according to a publication released in 2002 by the Purple Mountain Observatory, China, the dates 28.9.2057, 4.9.2089, and 7.8.2097 were miscalculated to the next day, changing the lengths of the corresponding months. The New Moon point on 28.9.2057 fell at 23:59, the New Moon point on 4.9.2089 fell at 23:57, and the New Moon point on 7.8.2097 fell at 23:59.
Therefore, statistics on full and deficient twelfth months in different years are not very meaningful, as everything depends on accurate calculation at each specific time.
Examples of full and deficient twelfth months
Mr. Binh also says that the years 2014, 2015, 2017, 2018, 2019, 2020, and 2021 had twelfth months with 30 days. But in 2022 the twelfth month was deficient with 29 days. In 2023 and 2024 it was full again. Then deficient twelfth months continue until 2033, when a full twelfth month appears again.
According to the Ten Thousand Year Calendar of the 20th-21st Centuries by Tran Tien Binh, the lunar calendar in 2024 has the 30th day of the twelfth month of Quy Mao, which is the 30th day of Tet Giap Thin. However, after that until 2032, the twelfth month is deficient every year. It is not until 2033 that the 30th day of the twelfth month appears again.
Why is the Vietnamese calendar sometimes different from the Chinese calendar?
According to Thanh Nien, the method of calculating the lunar calendar in Vietnam is the same as in China, both applying the same rules. The first day of the month is the New Moon day, a normal year has 12 months, a leap year has 13 months, the winter solstice always falls in the eleventh month, in a leap year the month without a mid-term is the leap month, this month is numbered with the month before it and the word leap is added. If in a leap year there are two months without a mid-term, the first month after the winter solstice 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 time zones 7 and 8.
Thus, the Vietnamese calendar is sometimes different from the Chinese calendar because of the last point above. This is based on Decision No. 121-CP dated 8.8.1967 signed by the late Prime Minister Pham Van Dong, stipulating that our country's official time zone is time zone 7 and the lunar calendar used in Vietnam is calculated according to this official time zone.
For example, in 1968, Vietnam began using the calendar calculated by time zone 7. Therefore, Vietnam celebrated Tet 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 New Moon point occurred at 23:29 Vietnam time on 29.1.1968, so the twelfth month in the Vietnamese calendar had only 29 days, a deficient month. At this time, according to Beijing time, China was already at 0:29 on 30.1, a full month. Therefore, the first day of Tet Nguyen Dan of Mau Than 1968 in Vietnam was one day earlier than in China.
Other lunisolar calendars in the world
According to Dan Tri, Mr. Tran Tien Binh adds that besides the East Asian lunisolar calendar 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 that follows only the moon's phases is the Islamic calendar. Accordingly, the first day of the year 2025 by the Islamic calendar falls around July 8 or 9 of the solar calendar.
In our country, by habit it is still called the lunar calendar and Lunar New Year. But when teaching students, it is necessary to state it fully and accurately.
Tet Nguyen Dan and how it is named
According to VietNamNet, Tet Nguyen Dan is also called Tet Ca, meaning the greatest festival of the year, Tet Ta to distinguish it from Western Tet, Lunar New Year to distinguish it from the solar calendar, traditional Tet, and today it is briefly called Tet.
According to Professor, Doctor of Science Tran Ngoc Them, Vietnam in particular and East Asian countries in general belong to the wet-rice agricultural civilization. Because agricultural cultivation depends greatly on nature, ancient people divided the time in a year into 24 solar terms to conveniently calculate the cycle of sowing and harvesting. The most important is the starting term of a cultivation and planting cycle, that is, the Nguyen Dan term, later mispronounced as Tet Nguyen Dan.
Japan is an Asian country that has abandoned Tet Nguyen Dan. Ganjitsu, Japan's traditional New Year today, falls on January 1, the first day of the solar year. Thus, this country celebrates New Year about one month earlier than neighboring countries. Although it does not organize Tet Nguyen Dan, traditional customs and festivals are still maintained by Japanese people during the solar New Year, such as lion dancing, hanging red lanterns, cooking traditional dishes, and commemorative parades.
Conclusion
Shortening the lunisolar calendar to lunar calendar is a habit deeply rooted in Vietnamese life. However, scientifically, this name does not accurately reflect the nature of the calendar being used. The Vietnamese calendar is a lunisolar calendar, combining the moon's phases and the Earth's movement around the Sun, with a leap month mechanism to adjust it to the four seasons. Understanding this nature correctly helps us not only know more about calendar science but also see the sophistication of ancient people in observing astronomy and applying it to agricultural life.
