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The Chinese Calendar


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 exception, 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.


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 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 "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 = 1091.5 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) 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

This still involved an error of 7.25 days in 19 years, or over a third of a day per year.

Date: 2015-01-29; view: 950

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