• Principal Term 1 occurs when the sun's longitude is 330 degrees.

• Principal Term 2 occurs when the sun's longitude is 0 degrees.

• Principal Term 3 occurs when the sun's longitude is 30 degrees.
  

• Principal Term 11 occurs when the sun's longitude is 270 degrees.

• Principal Term 12 occurs when the sun's longitude is 300 degrees.

Each month carries the number of the Principal Term that occurs in that month.

In rare cases, a month may contain two Principal Terms; in this case the months numbers may have to be shifted. Principal Term 11 (Winter Solstice) must always fall in the 11th month.

All the astronomical calculations are carried out for the meridian 120 degrees east of Greenwich. This roughly corresponds to the east coast of China.

Some variations in these rules are seen in various Chinese communities.

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What Years Are Leap Years?

Leap years have 13 months. To determine if a year is a leap year, calculate the number of new moons between the 11th month in one year (i.e., the month containing the Winter Solstice) and the 11th month in the following year. If there are 13 new moons from the start of the 11th month in the first year to the start of the 11th month in the second year, a leap month must be inserted.

In leap years, at least one month does not contain a Principal Term. The first such month is the leap month. It carries the same number as the previous month, with the additional note that it is the leap month.

How Does One Count Years?

Unlike most other calendars, the Chinese calendar does not count years in an infinite sequence. Instead years have names that are repeated every 60 years.

(Historically, years used to be counted since the accession of an emperor, but this was abolished after the 1911 revolution.)

Within each 60-year cycle, each year is assigned name consisting of two components:

The first component is a Celestial Stemm. These words have no English equivalent:

1.	jia	6.	ji
2.	yi	7.	geng
3.	bing	8.	xin
4.	ding	9.	ren
5.	wu	10.	gui

The second component is a Terrestrial Branch. The names of the corresponding animals in the zodiac cycle of 12 animals are given in parentheses.

1.	zi (rat)	7.	wu (horse)
2.	chou (ox)	8.	wei (sheep)
3.	yin (tiger)	9.	shen (monkey)
4.	mao (hare, rabbit)	10.	you (rooster)
5.	chen (dragon)	11.	xu (dog)
6.	si (snake)	12.	hai (pig)

Each of the two components is used sequentially. Thus, the 1st year of the 60-year cycle becomes jia-zi, the 2nd year is yi-chou, the 3rd year is bing-yin, etc. When we reach the end of a component, we start from the beginning: The 10th year is gui-you, the 11th year is jia-xu (restarting the Celestial Stem), the 12th year is yi-hai, and the 13th year is bing-zi (restarting the Terrestrial Branch). Finally, the 60th year becomes gui-hai.

This way of naming years within a 60-year cycle goes back approximately 2000 years. A similar naming of days and months has fallen into disuse, but the date name is still listed in calendars.

It is customary to number the 60-year cycles since 2637 B.C.E., when the calendar was supposedly invented. In that year the first 60-year cycle started.

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What Is the Current Year in the Chinese Calendar?

The current 60-year cycle started on 2 Feb 1984. That date bears the name bing-yin in the 60-day cycle, and the first month of that first year bears the name gui-chou in the 60-month cycle.

This means that the year wu-yin, the 15th year in the 78th cycle, started on 28 Jan 1998. The 20th year in the 78th cycle, started on 1 Feb 2003.

The following are dates for Chinese/Lunar New Year's day:

Chinese year	Zodiac animal	Gregorian calendar
4693	Boar	January 31, 1995
4694	Rat	February 19, 1996
4695	Ox	February 7, 1997
4696	Tiger	January 28, 1998
4697	Hare/Rabbit	February 16, 1999
4698	Dragon	February 5, 2000
4699	Snake	January 24, 2001
4700	Horse	February 12, 2002
4701	Ram/Sheep	February 1, 2003
4702	Monkey	January 22, 2004
4703	Rooster	February 9, 2005
4704	Dog	January 29, 2006
4705	Boar	February 18, 2007
4706	Rat	February 7, 2008
4707	Ox	January 26, 2009
4708	Tiger	February 10, 2010
4709	Hare/Rabbit	February 3, 2011
4710	Dragon	January 23, 2012
4711	Snake	February 10, 2013
4712	Horse	January 31, 2014
4713	Ram/Sheep	February 19, 2015
4714	Monkey	February 9, 2016
4715	Rooster	January 28, 2017
4716	Dog	February 16, 2018
4717	Boar	February 5, 2019
4718	Rat	January 25, 2020

What about the year 2033?

In the early 1990s, Chinese astronomers discovered that there was an error in the Chinese calendar for 2033. The traditional calendar claimed that the leap month would follow the 7th month, while in fact it comes after the 11th month. It is very unusual that the 11th month has a leap month, in fact it hasn't happened since the calendar reform in 1645 (before 1645, all months had the same probability for having a leap month). But many Chinese astronomers still claim that there will never be a leap month after the 12th and 1st month. In addition, there will be a leap month after the 1st month in 2262 (in fact, it should have happened in 1651, but they got the calculations wrong!) and there will be a leap month after the 12th month in 3358. Since the Chinese calendar is an astronomical calendar, predictions require delicate astronomical calculations, so my computations for 3358 should probably be taken with a grain of salt. 

When did the calendar really start?

If the Chinese calendar started in 2637 B.C.E., why is the current year 60 years too late? (e.g., in 1999, the current year was 4697? and not 4637)?

The Chinese calendar does not use a continuous year count! They used a 60 year cycle and a system of regional years (starting with each emperor). Before the 1911 revolution, Sun Yat-sen wanted to establish a republican alternative to the imperial reign cycles. According to Chinese tradition, the first year of the Yellow Emperor was 2698 B.C.E., so he introduced a counting system based on this. Under this system, 2000 is year 4698. An alternative system is to start with the first historical record of the 60-day cycle from March 8, 2637 B.C.E. Based on this system, 2000 is year 4637. 

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What was the Early Chinese calendar?

In China, the calendar was a sacred document, sponsored and promulgated by the reigning monarch. For more than two millennia, a Bureau of Astronomy made astronomical observations, calculated astronomical events such as eclipses, prepared astrological predictions, and maintained the calendar. After all, a successful calendar not only served practical needs, but also confirmed the consonance between Heaven and the imperial court.

Analysis of surviving astronomical records inscribed on oracle bones reveals a Chinese lunisolar calendar, with intercalation of lunar months, dating back to the Shang dynasty of the fourteenth century B.C.E. Various intercalation schemes were developed for the early calendars, including the nineteen-year and 76-year lunar phase cycles that came to be known in the West as the Metonic cycle and Callipic cycle.

From the earliest records, the beginning of the year occurred at a New Moon near the winter solstice. The choice of month for beginning the civil year varied with time and place, however. In the late second century B.C.E., a calendar reform established the practice, which continues today, of requiring the winter solstice to occur in month 11. This reform also introduced the intercalation system in which dates of New Moons are compared with the 24 solar terms. However, calculations were based on the mean motions resulting from the cyclic relationships. Inequalities in the Moon's motions were incorporated as early as the seventh century C.E., but the Sun's mean longitude was used for calculating the solar terms until 1644.

Years were counted from a succession of eras established by reigning emperors. Although the accession of an emperor would mark a new era, an emperor might also declare a new era at various times within his reign. The introduction of a new era was an attempt to reestablish a broken connection between Heaven and Earth, as personified by the emperor. The break might be revealed by the death of an emperor, the occurrence of a natural disaster, or the failure of astronomers to predict a celestial event such as an eclipse. In the latter case, a new era might mark the introduction of new astronomical or calendrical models.

Sexagenary cycles were used to count years, months, days, and fractions of a day using the set of Celestial Stems and Terrestrial Branches. Use of the sixty-day cycle is seen in the earliest astronomical records. By contrast the sixty-year cycle was introduced in the first century C.E. or possibly a century earlier. Although the day count has fallen into disuse in everyday life, it is still tabulated in calendars. The initial year (jia-zi) of the current year cycle began on 1984 February 2, which is the third day (bing-yin) of the day cycle.

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Details of early calendars

One of the two methods that they used to make this calendar was to add an extra month of 29 or 30 days, which they termed the 13th month, to the end of a regular 12-month year. There is also evidence that suggests that the Chinese developed the Metonic cycle (see above Complex cycles) -- i.e., 19 years with a total of 235 months--a century ahead of Meton's first calculation (no later than the Spring and Autumn period, 770-476 BC). During this cycle of 19 years there were seven intercalations of months. The other method, which was abandoned soon after the Shang started to adopt it, was to insert an extra month between any two months of a regular year. Possibly, a lack of astronomical and arithmetical knowledge allowed them to do this.


By the 3rd century BC, the first method of intercalation was gradually falling into disfavor, while the establishment of the meteorological cycle, the erh-shih-ssu chieh-ch'i (Pinyin ershisi jieqi), during this period officially revised the second method. This meteorological cycle contained 24 points, each beginning one of the periods named consecutively the Spring Begins, the Rain Water, the Excited Insects, the Vernal Equinox, the Clear and Bright, the Grain Rains, the Summer Begins, the Grain Fills, the Grain in Ear, the Summer Solstice, the Slight Heat, the Great Heat, the Autumn Begins, the Limit of Heat, the White Dew, the Autumn Equinox, the Cold Dew, the Hoar Frost Descends, the Winter Begins, the Little Snow, the Heavy Snow, the Winter Solstice, the Little Cold, and the Severe Cold. The establishment of this cycle required a fair amount of astronomical understanding of the Earth as a celestial body, and without elaborate equipment it is impossible to collect the necessary information. Modern scholars acknowledge the superiority of pre-Sung Chinese astronomy (at least until about the 13th century AD) over that of other, contemporary nations.


The 24 points within the meteorological cycle coincide with points 15º apart on the ecliptic (the plane of the Earth's yearly journey around the Sun or, if it is thought that the Sun turns around the Earth, the apparent journey of the Sun against the stars). It takes about 15.2 days for the Sun to travel from one of these points to another (because the ecliptic is a complete circle of 360º), and the Sun needs 365 1 /4 days to finish its journey in this cycle. Supposedly, each of the 12 months of the year contains two points, but, because a lunar month has only 29 1 /2 days and the two points share about 30.4 days, there is always the chance that a lunar month will fail to contain both points, though the distance between any two given points is only 15º. If such an occasion occurs, the intercalation of an extra month takes place. For instance, one may find a year with two "Julys" or with two "Augusts" in the Chinese calendar. In fact, the exact length of the month in the Chinese calendar is either 30 days or 29 days--a phenomenon which reflects its lunar origin. Also, the meteorological cycle means essentially a solar year. The Chinese thus consider their calendar as yin-yang li, or a "lunar-solar calendar."


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When were foreign calendars introduced?

Although the yin-yang li has been continuously employed by the Chinese, foreign calendars were introduced to the Chinese, the Hindu calendar, for instance, during the T'ang (Tang) dynasty (618-907), and were once used concurrently with the native calendar. This situation also held true for the Muslim calendar, which was introduced during the Yüan dynasty (1206-1368). The Gregorian calendar was taken to China by Jesuit missionaries in 1582, the very year that it was first used by Europeans. Not until 1912, after the general public adopted the Gregorian calendar, did the yin-yang li lose its primary importance.


Western (pre-Copernican) astronomical theories were introduced to China by Jesuit missionaries in the seventeenth century. Gradually, more modern Western concepts became known. Following the revolution of 1911, the traditional practice of counting years from the accession of an emperor was abolished.