Horology, or chronometry, refers to the measurement of time. In the context of the Chinese calendar, horology involves the definition and mathematical measurement of terms or elements such observable astronomic movements or events such as are associated with days, months, years, hours, and so on. These measurements are based upon objective, observable phenomena. Calendar accuracy is based upon accuracy and precision of measurements. The Chinese calendar is
lunisolar, similar to the
Hindu,
Hebrew and
ancient Babylonian calendars. In this case the calendar is in part based in objective, observable phenomena and in part by mathematical analysis to correlate the observed phenomena. Lunisolar calendars especially attempt to correlate the solar and lunar cycles, but other considerations can be agricultural and seasonal or phenological, or religious, or even political. Basic horological definitions include that days begin and end at midnight, and months begin on the day of the
new moon. Years start on the second (or third) new moon after the
winter solstice.
Solar terms govern the beginning, middle, and end of each month. A
sexagenary cycle, comprising the
heavenly stems () and the
earthly branches (), is used as identification alongside each year and month, including
intercalary months or leap months. Months are also annotated as either long ( for months with 30 days) or short ( for months with 29 days). There are also other elements of the traditional Chinese calendar.
Day Days are
Sun oriented, based upon divisions of the solar year. A day () is considered both traditionally and currently to be the time from one midnight to the next. Traditionally, days (including the night-time portion) were divided into 12 double-hours, and in modern times, the 24-hour system has become more standard.
Week As early as the Bronze Age
Xia dynasty, days were grouped into nine- or ten-day weeks known as . Months consisted of three . The first 10 days were the early (), the middle 10 the mid (), and the last nine (or 10) days were the late (). Japan
adopted this pattern, with 10-day-weeks known as . In Korea, they were known as
sun (,). The structure of led to
public holidays every five or ten days. Officials of the
Han dynasty were legally required to rest every five days (twice a , or 5–6 times a month). The name of these breaks became . Grouping days into sets of ten is still used today in referring to specific natural events. "Three Fu" (), a 29–30-day period which is the hottest of the year, reflects its three- length. After the winter solstice, nine sets of nine days were counted to calculate the end of winter.
Seven-day week and 28-day cycle The seven-day week was adopted from the Hellenistic system by the 4th century CE, although its method of transmission into China is unclear. It was again transmitted to China in the 8th century by
Manichaeans via
Kangju, spoken in a variant of
Sogdian language (a Central Asian kingdom near
Samarkand),. Its meaning is derived from the five classical planets, along with the Sun and Moon, making a total of seven celestial bodies highly visible in the sky, in Chinese translation 七曜. At that time, people created simple handwritten almanacs, where Sunday was marked with the character 密. The seven-day week had fallen out of favour for a long time, only to be revived when Christianity gained a foothold in China and later made mandatory by the government. In between, a 28-day cycle system was used, borrowing from the
Twenty-Eight Mansions system. Originally, these mansions tracked the moon's position against the stars in the sky, much like the sun and zodiac, and became part of the
Chinese constellations. However, in this context, the 28-day cycle had no connection to astronomy and was used purely for fortune-telling. This information was documented and is still referenced in the
Tung Shing, a Chinese almanac. When Westerners introduced the seven-day week system to China, whether for religious, business, or colonial reasons, both the Chinese and the Westerners found the 28-day cycle useful. Sunday, for instance, was written as "星房虛昴," indicating the corresponding four days on the 28-day cycle, as easily found in the almanac. Following the calendrical reforms in China during the era of the Republic of China, a period marked by both rejection and integration, the seven-day week system became the most widely used, aligning with the Western world.
Month Months are
Moon oriented.
Month (), the time from one
new moon to the next. These
synodic months are about days long. This includes the
Date (), when a day occurs in the month. Days are numbered in sequence from 1 to 29 (or 30). And, a
Calendar month (), is when a month occurs within a year. Some months may be repeated. Months are defined by the time between
new moons, which averages approximately days. There is no specified length of any particular Chinese month, so the first month could have 29 days (short month, ) in some years and 30 days (long month, ) in other years. Since the beginning of the month is determined by when the new moon occurs, other countries using this calendar use their own time standards to calculate it, resulting in deviations. The first new moon in 1968 was at 16:29
UTC on 29 January. Since
North Vietnam used
UTC+07:00 to calculate their
Vietnamese calendar and
South Vietnam used
UTC+08:00 (Beijing time) to calculate theirs, North Vietnam began the
Tết holiday at 29 January at 23:29 while South Vietnam began it on 30 January at 00:15. The time difference allowed asynchronous attacks in the
Tet Offensive. Because astronomical observation determines month length, dates on the calendar correspond to moon phases. The first day of each month is the new moon. On the seventh or eighth day of each month, the first-quarter moon is visible in the afternoon and early evening. On the 15th or 16th day of each month, the full moon is visible all night. On the 22nd or 23rd day of each month, the last-quarter moon is visible late at night and in the morning. Different eras used different systems to determine the length of each month. The
synodic month of the Taichu calendar was days long. The 7th-century, Tang-dynasty Wùyín Yuán Calendar was the first to determine month length by synodic month instead of the cycling method. Since then, month lengths have primarily been determined by observation and prediction. The days of the month are always written with two characters and numbered beginning with 1. Days one to 10 are written with the day's numeral, preceded by the character
Chū ();
Chūyī () is the first day of the month, and
Chūshí () the 10th. Days 11 to 20 are written as regular
Chinese numerals;
Shíwǔ () is the 15th day of the month, and
Èrshí () the 20th. Days 21 to 29 are written with the character
Niàn () before the characters one through nine;
Niànsān (), for example, is the 23rd day of the month. Day 30 (when applicable) is written as the numeral
Sānshí ().
Year A year () is based upon the time of one revolution of Earth around the Sun, rounded to whole days. Traditionally, the year is measured from the first day of spring (lunisolar year) or the winter solstice (solar year). A 12-month-year using this system has 354 days, which would drift significantly from the
tropical year. To fix this, traditional Chinese years have a 13-month year approximately once every three years. The 13-month version has the same long and short months alternating, but adds a 30-day leap month (). Years with 12 months are called common years, and 13-month years are known as long years. A solar year is astronomically about days. A lunisolar calendar year is either 353–355 or 383–385 days long. The lunisolar calendar () year usually begins on the new moon closest to
Lichun, the first day of spring. This is typically the second and sometimes third new moon after the winter solstice. The lunisolar year begins with the first spring month, (), and ends with the last winter month, (). All other months are named for their number in the month order. See
below on the timing of the Chinese New Year.
Solar year and solar terms The
solar year (), the time between
winter solstices, is divided into 24 periods. Each period corresponds to a 15°-portion of the ecliptic. The 24 moments of transition, known as ,
solar terms or solar nodes, mark off the seasons (both Western and Chinese), corresponding to equinoxes, solstices, and other Chinese events. The solar terms are pairedone corresponding to the sun being a multiple of 30° from a solstice, called "major" terms, or , and the other corresponding to the sun being 15° further west (earlier in the year), called "minor" terms. (The word jié qì can also be used to refer just to these.) The solar terms around 5 April and around 22 December are celebrated events in China. The solar year (
suì, ) begins on the December solstice and proceeds through the 24 solar terms. Since the speed of the Sun's apparent motion in the elliptical is variable, the time between major terms is not fixed. This variation in time between major terms results in different solar year lengths. There are generally 11 or 12 complete lunisolar months, plus two incomplete lunisolar months around the winter solstice, in a solar year. The complete lunisolar months are numbered from 0 to 10, and the lunisolar month split between two solar years is considered the 11th month. If there are 12 complete lunisolar months within a solar year, it is known as a leap year (a year possessing an intercalary month). Different versions of the traditional calendar might have different average solar year lengths. For example, one solar year of the 1st century BCE Tàichū calendar is (365.25016) days. A solar year of the 13th-century Shòushí calendar is (365.2425) days, identical to the Gregorian calendar. The additional .00766 day from the Tàichū calendar leads to a one-day shift every 130.5 years. If there are 12 complete lunisolar months within a solar year, the first lunisolar month that does not contain a major term is designated the
leap, or intercalary, month. (Quite rarely there is a year having a month that contains no major term but which is not counted as a leap month because there is another month later which contains two. This will happen in 2033.) Leap months are numbered with , the character for "intercalary", plus the name of the month they follow. In 2017, the intercalary month after month six was called , or "intercalary sixth month" () and written as
6i or
6+. The next intercalary month (in 2020, after month four) is called () and written
4i or
4+. Leap months occur on average once in years, or months (because of the
Metonic cycle of 19 years equaling 235 months), but this varies because the speed of the moon, and of the sun, varies.
Planets The movements of the Sun, Moon,
Mercury,
Venus,
Mars,
Jupiter and
Saturn (sometimes known as the
seven luminaries) are the references for calendar calculations. • The distance between Mercury and the sun is less than 30° (the sun's height at
chénshí:, 8:00 to 10:00 am), so Mercury was sometimes called the "chen star" (); it is more commonly known as the "water star" (). • Venus appears at dawn and dusk and is known as the "bright star" () or "long star" (). • Mars looks like fire and occurs irregularly, and is known as the "fire star" ( or ). Mars is the punisher in Chinese mythology. When Mars is near
Antares (), it is a bad omen and can forecast an emperor's death or a chancellor's removal (). • Jupiter's revolution period is 11.86 years, so Jupiter is called the "age star" (); 30° of Jupiter's revolution is about a year on earth. • Saturn's revolution period is about 28 years. Known as the "guard star" (), Saturn guards one of the
28 Mansions every year.
Stars Big Dipper The
Big Dipper is the celestial compass, and its handle's direction indicates the season and month.
3 Enclosures and 28 Mansions The stars are divided into
Three Enclosures and
28 Mansions according to their location in the sky relative to
Ursa Minor, at the center. Each mansion is named with a character describing the shape of its principal
asterism. The Three Enclosures are
Purple Forbidden, (),
Supreme Palace (), and
Heavenly Market (). The eastern mansions are . Southern mansions are . Western mansions are . Northern mansions are . The moon moves through about one
lunar mansion per day, so the 28 mansions were also used to count days. In the
Tang dynasty, Yuan Tiangang () matched the 28 mansions, seven luminaries and yearly animal signs to yield combinations such as "horn-wood-flood dragon" ().
List of lunar mansions The names and determinative stars of the mansions are: == Sexagenary system==