How to calculate the leap month in the lunar calendar?

In order to calculate the leap month as well as the lunar calendar in general, humans must determine the Soc points and the midpoint points, that is, to calculate the exact location of the Moon as well as the Earth in orbit.

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Mr. Tran Tien Binh, a calendar researcher, worked at the Vietnam Academy of Science and Technology and sent VnExpress the article on how to calculate leap months in the year, and pointed out the common misconceptions in the calculation. calendar:

People often mention the first day of the sound, but not everyone knows the physical meaning of this day. Day One is the day when the Earth, the Moon and the Sun are in the order of alignment. The moon turned half-dark to the Earth (so people often say dark like night 30). This time is called Squirrel. If the next two Soc periods (after rounding to the date) are 29 days apart, the month is missing; 30 days apart is enough.

If the calendar includes only the identification of consecutive Soc points , then there will be a negative calendar like the Islamic calendar and this calendar will be deviated systematically from the weather year (solar calendar) because the total negative 12 months only The average is approximately 354.36 days.

The second September in the year is October 24 of the calendar.

To have a calendar not only to follow the moon, but also to the climate, the ancients included in the calendar the factors related to the Earth's position in orbit around the Sun (or the location of the Sun di turn on the Zodiac if viewed from the Earth). So the calendar we are using, is exactly the lunisolar calendar , instead of the lunar calendar, because the ancients for the Moon represent yin and the Sun is positive.

Factors related to the position of the Earth in orbit around the Sun are expressed through the concept called Gas, which is known as Air. There are 24 gases in the solar year corresponding to 24 Earth positions around the Sun, 24 gases including 12 gas fields and 12 alternating central gases such as spring equinox are central gas, followed by clarification of gas, succession. coming to the cup dance is the central .

Of the 24 gases, 12 gas chambers are specifically used to calculate the calendar, while the other 12 have only marked the weather and season of the year. 12 central air from East solstice (about 21/12 calendar) of this year to East only next year fit a rotation of the Earth around the Sun.

The ancients compared the 12 lunar months with 12 midpoints so that the lunar year did not deviate from the climate. If there are only 12 Soc points in the middle of the East and 12 negative months, that year there is no leap month. If there are 13 Soc points in this time period, there will be a negative negative month that does not correspond to any center and that month will be a leap month.

So calculating the Soc points, we know the days of the month, but to know which month it is, we must calculate the midfield to know if there is a leap month of the year. In addition, the negative November always contains the central air named Dong chi, which is the basis for numbering other months.

In other words, the exact first month that does not contain the center will be the leap month and the name will be the same as the previous month. For example, this year, from the winter to the winter of 2014, there are 13 negative months and the month after September is negative.

So in order to calculate the leap month as well as the lunar calendar in general, we have to identify the Soc points and the central gas points, that is to calculate the exact location of the Moon as well as the Earth in the orbital motion.

Why must the accuracy and not make the table or calculate the balance in the division? In the evidence above the winter solstice in 1984 occurred at 23h23 on December 21, Hanoi time, ie 0h23 on December 22, Beijing time. Due to one day deviation, between the two East points, only that year according to the Vietnamese calendar, there were only 12 Soc points, ie there was no leap month and the Chinese calendar was profitable in October.

The minute accuracy of Soc or neutral points is a mechanical problem not simple, especially in the case of the Moon being affected not only by the attraction of the Earth or the Sun but also of many other celestial bodies like Jupiter, Saturn .

It is also impossible to assume that the calendar calculation has been accurate for millions of years. When applying modern astronomical models to develop in recent decades, researchers have found that the Vietnamese calendar in 1997 was wrong in one place, that is the first day of the first day when the sound fell in 23h56 on December 29, not the date of December 30 as announced, but because it is the past, it is not reviewed. Even the Chinese calendar published by the Golden Dragon Observatory also has a deviation from the Naval Observatory data (USA).