Is January never a leap month?

On social networks, many people share information that the lunar leap month never falls in January, considering it a strange and interesting phenomenon.

MSc. Tran Tien Binh, former official of the State Calendar Committee, later the Calendar Research Department, Information and Documentation Center, Vietnam Academy of Science and Technology, author of the book"Vietnamese calendar 20th-21st century"denied this. He said that any lunar month of the year can be a leap month. However, January is very rarely a leap month and this leads to the above misconception.

In fact, the Vietnamese lunar calendar of the 20th - 21st centuries does not have a December or January leap month, so many people believe that January cannot be a leap month. However, going back further in time, we will see that the year of Quy Hoi (1803) had a leap month of January. In the 22nd century, the phenomenon of a leap month of January appeared in the year of Mau Than - 2148. That year, there will be 2 January 1st days: Sunday, January 21, 2148 and Tuesday, February 20, 2148.

Similarly, many people also think that the 11th lunar month cannot be a leap month. However, the calendar year 2033 will have 2 11th lunar months.

Explaining how to calculate the leap month, Master Tran Tien Binh said that a lunar month has exactly 29.53 days, however, the lunar year is rounded to 354 days, 11 days less than a solar year, and every 3 years is 33 days less.

To balance the time between these two types of calendars, calendar makers calculate that every 3 lunar years, a leap month must be added. However, even with a leap month, the lunar year is always slower than the solar year. To overcome this situation, they agreed that every 19 years, a leap month must be added. A leap year is a year with 13 lunar months between the 2 winter solstices (around December 21st of the solar calendar, always falling in the 11th lunar month).

How do you determine which month is an intercalary month? Calendar makers calculate the Middle Qi points. Between the two winter solstices, there are 12 Middle Qi points: Winter solstice, Great cold, Rain water, Spring equinox, Grain rain, Little fullness, Summer solstice, Great heat, Chu heat, Autumn equinox, Frostfall and Little snow. These points are usually about 1 month apart.

A leap lunar year has 13 months but the number of Trung Khi points is always only 12, so in a leap year there will be months that do not contain any Trung Khi points. Any month that does not contain a Trung Khi point is a leap month, with the same name as the previous lunar month.

However, the calculation of leap years also needs to be observed based on actual changes. Because the Soc points (the time when the moon is not there) used to determine the first day of the lunar calendar and the Trung Khi points need to be recalculated very accurately, sometimes only a few minutes off to another day and this affects the determination of the leap month.

For example, according to Vietnam time, the winter solstice point in 1984 was at 23:23 on December 21 and according to China time it was 0:23 on December 22; the spring equinox point in Vietnam was 23:14 on March 20, 1985 and in China the spring equinox was 0:14 on March 21, 1985 (due to different time zones, Vietnam is GMT+7, and China is GMT+8).

Therefore, the Chinese lunar calendar had a leap month of October in 1984, while the Vietnamese lunar calendar had a leap month of February in 1985; causing the Lunar New Year between the two calendars to differ by 1 month.

Returning to the issue of the rare leap month of January, Master Tran Tien Binh affirmed that this is due to the operation of the calendar according to conventional calculations, not the subjective will of calendar makers.