Rules of Chinese Calendar

The following rules have been valid since 104 BC, although some of the details were unnecessary before 1645. Note that these rules do not specify the detailed calculations, permitting either mean or true motions of the Sun and Moon to be used, depending on the historical period.

1. The months are lunar months, such that the first day of each month beginning at midnight is the day of the astronomical new moon.
   
2. Each year has 12 regular months, which are numbered in sequence (1 to 12)and have alternative names. Every second or third year has an intercalary month (runyue), which may come after any regular month. It has the same number as the preceding regular month, but is designated intercalary.
   
3. Every other jiéqì of the Chinese solar year is equivalent to an entry of the sun into a sign of the tropical zodiac (a principle term or cusp).
   
4. The sun always passes the winter solstice (enters Capricorn) during month 11.
   
5. If there are 12 months between two successive occurrences of month 11, at least one of these 12 months must be a month during which the sun remains within the same zodiac sign throughout (no principle term or cusp occurs within it). If only one such month occurs, it is designated intercalary, but if two such months occur, only the first is designated intercalary.
   
6. The times of the astronomical new moons and the sun entering a zodiac sign are determined in the Chinese Time Zone by the Purple Mountain Observatory (zijinshan tianwent) outside Nanjing using modern astronomical equations.

The Zodiac Sign which the sun enters during the month and the ecliptic longitude of that entry point usually determine the number of a regular month. Month 1, zhēngyuè, literally means principal month. All other months are literally numbered, second month, third month, etc.

Some believe the above correspondence to be always true, but there are exceptions, which, for example, prevent Chinese New Year from always being the second new moon after the winter solstice, or that cause the holiday to occur after the Rain Water jieqi. An exception will occur in 2033-2034, when the winter solstice is the second solar term in the eleventh month. The next month is a no-entry month and so is intercalary, and a twelfth month follows which contains both the Aquarius and Pisces solar terms (deep cold and rain water). The Year of the Tiger thus begins on the third new moon following the Winter Solstice, and also occurs after the Pisces (rain water) jieqi, on February 19.

Another occurrence was in 1984-85, after the sun had entered both Capricorn at 270° and Aquarius at 300° in month 11, and then entered Pisces at 330° during the next month, which should have caused it to be month 1. The sun did not enter any sign during the next month. In order to keep the winter solstice in month 11, the month which should have been month 1 became month 12, and the month thereafter became month 1, causing Chinese New Year to occur on 20 February 1985 after the sun had already passed into Pisces at 330° during the previous month, rather than during the month beginning on that day.

On those occasions when a dual-entry month does occur, it always occurs somewhere between two months that do not have any entry (non-entry months). It usually occurs alone and either includes the winter solstice or is nearby, thus placing the winter solstice in month 11 (rule 4) chooses which of the two non-entry months becomes the intercalary month. In 1984-85, the month immediately before the dual-entry month 11 was a non-entry month which was designated as an intercalary month 10. All months from the dual-entry month to the non-entry month that is not to be intercalary are sequentially numbered with the nearby regular months (rule 2). The last phrase of rule 5, choosing the first of two non-entry months between months 11, has not been required since the last calendar reform, and will not be necessary until the 2033-34 occasion, when two dual-entry months will be interspersed among three non-entry months, two of which will be on one side of month 11. The leap eleventh month produced is a very rare occasion. See [3] for details.

Exceptions such as these are rare. Fully 96.6% of all months contain only one entry into a zodiacal sign (have one principle term or cusp), all obeying the numbering rules of the jiéqì table, and 3.0% of all months are intercalary months (always non-entry months between principle terms or cusps). Only 0.4% of all months either are dual-entry months (have two principle terms or cusps) or are neighboring months that are renumbered.

It is only after the 1645 reform that this situation arose. Then it became necessary to fix one month to always contain its principal term and allow any other to occasionally not contain its principal term. Month 11 was chosen, because its principal term (the winter solstice) forms the start of the Chinese Solar year (the sui).

The Chinese lunar calendar and the Gregorian Calendar often sync up every 19 years (Metonic cycle). Most Chinese people notice that their Chinese and Western birthdays often fall on the same day on their 19th, 38th birthday etc. However, a 19-year cycle with an certain set of intercalary months is only an approximation, so an almost identical pattern of intercalary months in subsequent cycles will eventually change after some multiple of 19 years to a quite different 19-year cycle.

The Chinese zodiac (see Nomenclature and Twelve Animals sections) is only used in naming years—it is not used in the actual calculation of the calendar. In fact, the Chinese have a very different constellation system.

The twelve months are closely connected with agriculture, so they are alternatively named after plants:

1. Primens (first month): Latin "primus mensis".
2. Apricomens (apricot month): apricot blossoms.
3. Peacimens (peach month): peach blossoms.
4. Plumens (plum month): plum ripens.
5. Guavamens (guava month): guava blossoms.
6. Lotumens (lotus month): lotus blossoms.
7. Orchimens (orchid month): orchid blossoms.
8. Osmanthumens (osmanthus month): osmanthus blossoms.
9. Chrysanthemens (chrysanthemum month): chrysanthemum blossoms.
10. Benimens (good month): good month.
11. Hiemens (hiemal month): hiemal month.
12. Lamens (last month): last month.