Ask someone in China what the date is and you can get two answers. One comes from the Gregorian calendar, which runs work, school, and flights. The other comes from the Chinese lunar calendar, a system thousands of years old, and it is the one that decides when Lunar New Year falls and when families sit down for the Mid-Autumn Festival. Most people use it without knowing how it was built.
The Chinese lunar calendar counts months by the moon, and the months drift
The Gregorian calendar counts the sun and lets the months fall where they may. The Chinese calendar counts the moon: a new month starts at each new moon, so the date tells you the moon’s phase. Look up on the fifteenth of any Chinese month and the moon is full. That is why the Mid-Autumn Festival, a moon-viewing holiday, falls on the fifteenth.
Twelve lunar months come to about 354 days, roughly eleven short of a solar year. Ignore the gap and it compounds. Within a decade the month that once held the spring planting has slid into winter, and a calendar that cannot tell a farmer when to sow is worthless.
An extra month every few years pulls the calendar back into place
Western calendars fix a similar gap with a leap day. Chinese calendar-makers needed to make up eleven days a year, so they inserted a whole leap month, roughly seven times every nineteen years. A Chinese leap year has thirteen months in it.
The leap month is what holds the two halves together. Astronomers call the result a lunisolar calendar. Chinese speakers usually call it the yinli, the lunar calendar, which credits only half the machinery.
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The system reached its mature form at the court of Emperor Wu of Han, during the Western Han dynasty. The Taichu calendar issued there fixed the astronomical positions and set the leap month rule that Chinese calendar-makers used for the next two thousand years. Later dynasties sharpened the arithmetic and left the framework alone.
The 24 solar terms follow the sun and land on the same dates each year
The Chinese calendar divides the solar year into 24 named segments called solar terms, each one marking the sun’s arrival at a fixed point along its yearly path: 360 degrees cut into 24 equal steps of 15 degrees.
The solstices and the equinoxes are among them. The other twenty fill in the gaps with names that read like weather reports, because that is what they are. Start of Spring. Rain Water. Awakening of Insects. Grain Rain.
Because they follow the sun, they land on nearly the same Gregorian date every year, give or take a day. They tell a farmer when the ground thaws, when the rains thicken, when grain ripens, and when it is time to store the harvest. Two thousand years before weather forecasting, this was the forecast.
Chinese has a phrase for the principle, shun tian shi, moving with the timing of heaven. Plow in spring, weed in summer, harvest in autumn, store in winter.
The almanac added lucky days, animal years, and star charts
The calendar is the astronomy. The huangli, the traditional almanac printed on top of it, is the household version, and it works much like The Old Farmer’s Almanac in the United States.
The imperial court issued a fresh calendar every year, and as those volumes reached ordinary readers, printers packed in whatever people wanted to look up. A traditional huangli lists the solar terms, the moon’s phases, a 28-sector map of the night sky used to judge the season, the sixty-year cycle of heavenly stems and earthly branches that gives each year its animal and element, and a daily column of activities marked suitable or unsuitable.
Spring pushes life outward and suits beginnings; autumn draws it back and suits storing. Picking an auspicious day for a wedding, a burial, or a shop opening was an attempt to match the plan to the direction the season was already heading.
Japan dropped the calendar in 1873 and kept the seasonal markers
Japan, Korea, and Vietnam all ran on the Chinese calendar for centuries and each built local variants on it.
Original article
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