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The Chinese Calendar

Compare: Five Elements, Heaven Stems & Earth Branches,
Eight Trigrams

Introduction

In the English-speaking world we use the Gregorian calendar, a 1582 modification (named after Pope Gregory XIII) of the "Julian" calendar established by Julius Caesar in ancient Rome. Although many other calendars have existed through history, and others are used in large parts of the world today, the Gregorian calendar, complete with months derived from their Latin names and with its peculiar "rump" month of February, is now the international standard.

China uses two calendars, one lunar and the other the Gregorian, often referred to as yin and yang calendars, respectively, or as the "agricultural calendar" and the "national calendar." There is also a traditional Chinese solar calendar, different from the Gregorian calendar, as we shall see.

With one significant exception, nearly all traditional Chinese festivals are based on the lunar calendar. So traditionally were markets, court sessions, temple fairs, and all private agreements to meet to do business.

History

In general, a lunar calendar, wherein a month corresponds to the cycle of phases of the moon, makes sense in a society where there is little artificial lighting, and the presence or absence of a bright moon makes a big difference to nocturnal activity (including, I can attest from experience, making it to the outhouse without mishap!).

On the other hand, a solar calendar, with the year anchored to the solstices and equinoxes, more realistically reflects our experience with seasons, and facilitates discussing longer-term historical phenomena (like how old people are, or when the mortgage will need to be paid off).

By the middle of the 2nd millennium BC, Chinese observers had concluded that the solar year was pretty nearly 365.25 days long. (The actual length is a hair shorter, which is why in the international Gregorian calendar, although we create a Leap Year by adding February 29 in years equally divisible by 4, we skip Leap Year in centennial years, unless they are equally divisible by 400.)

Each cycle of the moon is very close to 29.5 days long. To accommodate the half day, some Chinese months are 29 days long and some 30 days long. That part was easy. The hard part came (as in all calendars) in trying to make lunations fit the length of the solar year:

1 year = 365 days
12 lunar months = 29.5 x 12 = 354 days (11 days short per year)

In other words, there are (365.25 ÷ 29.5 = ) 12.3813559322… lunar months per year. That is not a very felicitous number if you want to make a calendar that fits the movement of both celestial bodies. (In our own calendar we ignore this problem and let the moon go through its phases without regard to the days of our artificial and arbitrary "months.")

Since each solar year is about a third of a lunar month longer than 12 lunar months, one could imagine reducing the error by adding an extra month each third year:

3 years = 365.25 x 3 days = 1,095.75 days
37 months = 29.5 x 37 = 1,091.5 days
difference = 4.25 days in three years, 1.4167 days per year)

That is still a relatively large error. The problem was partially solved, probably by about the Spring & Autumn Period (770-476 BC) of the Eastern Zhōu 周 dynasty by using a cycle of 19 years, in seven of which intercalary months were inserted:

19 years = 365.25 days = 6,939.75 days (6,935 if one ignores the quarter days)
19 years x 12 months = 228 months, plus 7 intercalary months = 235 months
235 months x 29.5 days = 6,932.5 days

Ingenious as this was, it still involved an error of 7.25 days in 19 years, or over a third of a day per year.

Chinese Lunar & Solar Calendars & Intercalary Months

The modern Chinese lunar calendar, which seems to have developed sometime in the third century BC, still designates some months as long (30 days) and some as short (29) days. But this is linked to the Chinese solar calendar, which of course does not quite correspond to the Gregorian calendar. And this brings us to the Chinese solar calendar.

The Chinese solar calendar is based on the movement of the sun over 24 named points 15 degrees apart on the 360-degree solar ecliptic. (The points are usually called "solar terms" in English, jiéqì节气 in Chinese, and each has a name, typically referring to the north Chinese agricultural cycle. The names of the terms are given at the bottom of this page [link]. Each of them falls within a day or so of the same date in the Gregorian calendar each year.)

Solar movement over the ecliptic is such that the points are 15.2 days apart (total 364.8 days). Now here is the ingenious part: It takes the sun (15.2 x 2 =) 30.4 days to move from one solar term, across the next, and land on the one after that. That is slightly longer than a "long" lunar month of 30 days. Therefore, whereas most lunar months will contain two solar points, a few lunar months will contain only one. This is what triggers the insertion of an immediate additional, "intercalary" lunar month (rùnyuè 闰月). (Actually, it is a bit more complicated that that. If you really want the details —you don't— click here.)

Lunar months are numbered rather than named. Although the intercalary month receives the same number as the preceding month (plus the prefixed character rùn 闰), no festivals associated with that month are repeated. Indeed, intercalary months have a reputation for being rather dreary, and some people even think of them as being generally times of bad luck.

The effect of inserting the intercalary months based on the error between the lunar and the solar cycles is to provide a constant correction for the misfit between the two calendars. This device has kept the lunar calendar reasonably well linked both to the phases of the moon and to the real solar year for something over two millennia.

Perhaps because of its associations with the workings of the cosmos, calculation of the calendar was an imperial government prerogative until the XXth century, and working it out for yourself and publishing your own calendar was considered an act of treason.

Festivals

Chinese New Year falls on the first day of the first lunar month. The insertion of intercalary months is the reason why Chinese New Year, like other traditional Chinese festivals, does not correspond with the same Gregorian date each year. It is the lunar calendar which determines the celebration of nearly all festival days.

Only one significant Chinese festival, "Clear and Bright" (Qīngmíng 清明), is based on the 24 solar terms. (For this reason most Chinese pay little or no attention to the solar terms, and I have heard ignorant Chinese high school students vigorously insist that Qīngmíng, being traditional, is therefore a lunar festival, even though they are unable to explain why it never seems to fall on the same lunar date.)

Galloping Gregorianization

Today only the Gregorian calendar is official in China, and Chinese tend to be so ambivalent about "old fashioned" lunar dates that they have yet to manufacture, say, a watch that shows lunar as well as solar dates. During the dark days of the "Cultural Revolution" in China, published calendars deliberately excluded lunar dates to avoid appearing to endorse traditional culture. However Chinese around the world continue to celebrate traditional holidays on the lunar calendar. (In Japan, which borrowed many of the same festivals from China, they have been shifted to solar dates.)

Historical Dates

Historical dates were (and in Chinese often still are) normally given as a dynasty name plus reign name plus year within the reign (counted from the first lunar new year in the reign), followed by the lunar month and the day of that month:

1903 July 24 =

清 Qīng dynasty

光绪 Guāngxù reign

二十九年 29th year

六月 6th lunar month

十一日 11th day

(Since all reign names begin on lunar new year, a month or two after solar new year, Chinese reign years do not perfectly correspond with Western years, although the error is small enough that it is easily ignored most of the time.)

In Taiwan the convention of using "reign names" continues, and dates after the end of the last imperial reign are normally given in years since the founding of the Republic of China in 1911, although the numbers change on solar new year (January 1) rather than lunar new year. (E.g., AD 2023 was 2023-1911 = ROC 112 or Mínguó 民國 112.)

Almanacs and Horoscopes

In addition to solar and lunar calendars, Chinese tradition provides for the continuous numbering of years, months, and days using a never-ending cycle of 60 two-character terms, each made of one of the ten "Heaven Stems" and one of the twelve "Earth Branches." This numbering naturally blocks years into cycles of sixty, which are continuously numbered, beginning from 2397 BC. Although previously dates like xīnmǎo 辛卯 (year number 28 in the cycle of 60) were commonly used in dates, today such designations are not actually used for calendrical purposes, but figure in calculations of the "astrological" qualities of each day (and are given on calendars). A separate page of this web site is devoted to the Heaven Stems and Earth Branches.

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