Applying the rules
Differences to Chinese lunar calendar
Vietnamese calendar rules
The Vietnamese calendar is a kind of astronomical calendar. It can be calculated for any arbitrary year based on the motion of the Sun, the Earth and the Moon. To be able to compute it for any time frame you basically need to compute 2 things:
- any New Moon, and
- the Principal Terms (trung khí) of any year
New Moon is a time the Sun, the Moon and the Earth are on the same line. It occurs once every about 29.5 days.
The 12 Principal Terms (also called Major Terms) are points that divide the ecliptic (đường hoàng đạo) into equal sectors. Four of them are also used as seasonal markers in Western calendar: March equinox (Xuân phân, around 20/3), June solstice (Hạ chí, around 22/6), September equinox (Thu phân, around 23/9), and December solstice (Đông chí, around 22/12).
The New Moon are used to determine when the lunar months begin and end. The Principal Terms are used to determine the leap months (intercalary months, tháng nhuận) and the names of the lunar months. Because they are based on solar positions, the Vietnamese calendar is not a pure lunar one, but a lunisolar calendar (âm dương lịch).
Once the New Moons and Major Terms are calculated, the following rules can be applied to determine the months and dates. The rules are adapted from the Explanatory Supplement to the Astronomical Almanac, P. Kenneth Seidelmann, editor:
1- The first day of the month is the day on which the New Moon occurs.
2- An ordinary year has twelve lunar months; an intercalary year has thirteen lunar months.
3- The Winter Solstice always falls in month 11.
4- In an intercalary year, a month in which there is no Principal Term is the intercalary month. It is assigned the number of the preceding month, with the further designation of intercalary. If two months of an intercalary year contain no Principal Term, only the first such month after the Winter Solstice is considered intercalary.
5- Calculations are based on the meridian 105° East.