Chinese Calendar - Calendar Rules

Calendar Rules

The following rules outline the Chinese calendar since 104 BC. Note: the rules allow either mean or true motions of the Sun and Moon to be used, depending on the historical period.

1. The months are lunar months. This means the first day of each month beginning at midnight is the day of the astronomical dark moon. (This differs from a traditional Chinese "day" which begins at 11 p.m.).
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 (traditional Chinese: 閏月; simplified Chinese: 闰月; pinyin: rùnyuè), 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 principal term or cusp).
4. The center of each Chinese solar month (the principal term or cusp) sets the name of that lunar month. Thus each Chinese solar month is shifted by 15° (half of each) from nearby Western zodiacal signs.
5. The sun always passes the winter solstice (enters Capricorn) during month 11.
6. If there are 12 months between two successive occurrences of month 11, not counting either 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 principal 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. Note that for calendars before true motions of the sun were used for naming (i.e., before 1645), or in years where there is no double-cusp month in that year or the previous or following years (i.e., usually), the following rule suffices: a month with no principal term (or cusp) in it is designated intercalary.
7. The times of the astronomical new moons and the sun entering a zodiac sign are determined using the time in the Chinese Time Zone by the Purple Mountain Observatory (Chinese: 紫金山天文台; pinyin: Zǐjīnshān Tiānwéntái) outside Nanjing using modern astronomical equations.