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. This it is linked to the Chinese solar calendar, which of course does not 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. The names of the terms are given at the bottom of this page. 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 lunar month. Therefore, whereas most lunar months will contain two solar points, a few lunar months will contain only one. This triggers the insertion of an immediate additional, "intercalary" lunar month (rùnyuè 闰月).
Lunar months are numbered rather than named. Although the intercalary month receives the same number as the preceding month (preceded by the character rùn 闰), no festivals associated with that month are repeated. Indeed, intercalary months have a reputation for being rather dreary, since there are no festivals at all associated with them, 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.
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.)
Today only the Gregorian calendar is official in China, and Chinese tend to be ambivalent enough 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 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 =
6th lunar month
(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 are normally given in years since the founding of the Republic of China, although the numbers change on solar new year (January 1). (E.g., AD 2000 was ROC 89 or Mínguó 民國 89.)
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.
Table of Solar Terms (Jiéqì 节气)
Approximate Gregorian Month & Day
Clear and Bright
Grain in Ear
Hoar Frost Falls
The 24 Solar Terms
The 24 solar terms is a gross name of the system that comprises of 12 major solar terms and 12 minor solar terms interlaced with each other. Starting from "vernal equinox", the 12 major solar terms are "vernal equinox", "corn rain", "corn forms", "summer solstice", "great heat", "end of heat", "autumnal equinox", "frost", "light snow", "winter solstice", "severe cold" and "spring showers". Each major solar term falls on one of the 12 lunar months designated by the 12 earthly branches (Figure 1). The minor solar term after "vernal equinox" is "bright and clear", and then in turn "summer commences", "corn on ear", "moderate heat", "autumn commences", "white dew", "cold dew", "winter commences", "heavy snow", "moderate cold", "spring commences" and "insects waken".
Figure 1 Tropical year, synodic months and the 24 solar terms.
From the Earth's perspective, the Sun moves through a year across the stars or celestial sphere along a path known as the ecliptic, which is measured in 360 degrees longitude. The 24 solar terms divide the ecliptic into 24 equal segments, with 15 degrees of the Sun's longitude between the terms. At "vernal equinox", the Sun's longitude is 0 degree; at "bright and clear", the Sun's longitude is 15 degrees; and so forth (Table 1).
Table 1 The 24 solar terms and the Sun's longitudes
Major solar term
Minor solar term
Bright and Clear
Corn on Ear
Major solar term
End of Heat
Minor solar term
Major solar term
Minor solar term
At "vernal equinox" and "autumnal equinox", the periods of daylight and the night are equal in length. The period of daylight is the longest at "summer solstice" and the shortest at "winter solstice" (northern hemisphere). These were the earliest solar terms determined in ancient time. Then it came the four solar terms "spring commences", "summer commences", "autumn commences" and "winter commences". Other solar terms were named later according to the weather and agricultural activities prevalent at the respective times of the seasons. The "24 solar terms" reflects to some extent the climate over central China in ancient time.
Each lunar month in the Agricultural Calendar contains a major solar term. A lunar month that does not include a major solar term is taken as the leap month of the preceding month. In 19 tropical years there will be 228 major solar terms and 235 synodic months. So 7 lunar months will not contain major solar terms and they are classified as leap months.
The 24 Solar Terms and the 12 Zodiac Constellations
Similar to the 24 solar terms, the 12 zodiac constellations were determined in the ancient time by dividing the zodiac (a band around the ecliptic) into 12 equal sectors measuring from the "vernal equinox" (Figure 2). Hence the start or end date of each zodiac sign in astrology always falls within about one day on a major solar term.
Figure 2 The 24 solar terms and the 12 zodiac constellations.
Remarks: The determination of the 12 zodiac constellations according to the Sun's longitude was made more than 2000 years ago. Due to precession of the Earth's rotation axis, the positions of the constellations as observed nowadays have already shifted to other longitudes.