The Theory of Chinese Lunar Calendar
How does the Chinese Lunar Calendar work?
Many people wonder how the Chinese Lunar Calendar works. Is there any mathematical or astronomical formula to convert the Gregorian calendar to the Chinese lunar calendar? The following explanation of the theory of the Chinese Lunar Calendar can answer these questions.
The Chinese calendar combines the lunar and solar systems. The Year and Day cycles use the solar system. However, there are two different Month cycles in the Chinese calendar. One uses the lunar system, and the other uses the solar system. In the lunar system of Month, the new moon day is the first day of a lunar month. The length of a lunar month is the length of time between two new moon days. The name of a lunar month is taken from the solar system. The Chinese solar months are not like the months of a modern calendar. The Chinese calendar divides the year into 24 solar segments 節氣 according to the sun's positions on the tropical zodiac (similar to western astrology). Each segment's name was given for ancient Chinese farmers' use. To understand the whole picture view the following chart.
Chinese Lunar Calendar - Baby Gender Chart
The first month of the year is Tiger month. The first day of Tiger Month is the Start of Spring, which is around February 4 each year.
| Solar Segment |
Sun Longitude |
Month Name |
Astrology Name |
|---|---|---|---|
| 立春 Start of Spring | 315 | 1 - 虎 Tiger | |
| 雨水 The Rains | 330 | ||
| 驚蟄 Excited Insects | 345 | 2 - 兔 Rabbit | |
| 春分 Vernal Equinox | 0 | ||
| 清明 Clear and Bright | 15 | 3 - 龍 Dragon | |
| 穀雨 Grain Rains | 30 | ||
| 立夏 Start of Summer | 45 | 4 - 蛇 Snake | |
| 小滿 Grain Fills | 60 | ||
| 芒種 Grain in Ear | 75 | 5 - 馬 Horse | |
| 夏至 Summer solstice | 90 | ||
| 小暑 Slight Heat | 105 | 6 - 羊 Goat | |
| 大暑 Great Heat | 120 | ||
| 立秋 Start of Autumn | 135 | 7 - 猴 Monkey | |
| 處暑 Still Hot | 150 | ||
| 白露 White Dew | 165 | 8 - 雞 Rooster | |
| 秋分 Autumnal Equinox | 180 | ||
| 寒露 Cold Dew | 195 | 9 - 狗 Dog | |
| 霜降 Frost Descends | 210 | ||
| 立冬 Start of Winter | 225 | 10 - 豬 Pig | |
| 小雪 Light Snow | 240 | ||
| 大雪 Heavy Snow | 255 | 11 - 鼠 Rat | |
| 冬至 Winter Solstice | 270 | ||
| 小寒 Little Cold | 285 | 12 - 牛 Ox | |
| 大寒 Severe Cold | 300 |
The solar months are defined by the sun's longitudes. The first month, Tiger Month, begins at the Start of Spring segment or when the sun enters the 315th degree on the tropical zodiac. The second month, Rabbit Month, begins from the Excited Insects segment or when the sun enters the 345th degree. The beginning of a solar month is called the 節氣 Segment Point. The middle point of a solar month is called the 中氣 Center Point . For example, the segments of Rain Water and Vernal Equinox are Center Points. The astrology names of Pisces and Aries apply to these Center Points.
The name of the lunar month is derived from the Solar Center Point. For example, if a lunar month contains the Rain Water Center Point, then it is the first month of the lunar calendar, also known as the Tiger month.
How does the Chinese Lunar Leap Month work?
If a lunar month does not contain any Solar Center Point, that month is known as a Leap Month . The name of this leap month is the same as the previous lunar month. The reason to assign leap months is that we want the moon cycle to harmonize with the sun cycle. Because one year has about 365.2425 days and one month has about 29.53 days, one year has about 12.3685 months. 12.3685 is about 12 and 7/19. If we put seven extra months in 19 years, the sun and moon should be back to the same sky position. "Seven Leap Months in 19 years" is easy for people to remember. However, 12.3685 is almost equal to 12 and 144/391. That means "144 Leap months in 391 years" is a more accurate way to assign Leap months. So far, no one has lived long enough to observe the entire cycle.
Anyway, the principles of the Chinese Calendar use the following four rules which the ancient Chinese made them thousand of years ago.
- The new moon day is the first day of a lunar month
- The name of a lunar name is named by the Solar Center Point
- The first lunar month of the year is the 虎 Tiger 寅 month, which contains Rain Water center point
- The lunar month without a Center Point is a Leap month ( 閏月 Intercalary Month )
In the Chinese Lunar Calendar, the new moon day is the first day of a lunar month and the length of a lunar month is the length between two new moon days. In the Chinese Fortune-Telling (Astrology) Calendar, the first day of a month is the day on a Solar Segment Point and the length of a month is the length between two Solar Segment Points. Therefore there is no Leap Month in the Chinese Fortune-Telling System.
Without this knowledge, above-mentioned, most people will be confused about the solar months and lunar months. They may think all Chinese Fortune-Telling systems use lunar months. Actually, the Fortune-Telling system called 八字 Eight Characters only uses the solar month, not the lunar month. The other popular system called 紫微斗數 Dipper uses solar month and lunar month together.
Stem-Branch is the Chinese calendar counting system
The counting system of Day is the 干支 Stem-Branch 60-day cycle system. No one knows who invented the Stem-Branch for the Day counting system, but legend has it that characters of Day Stem-Branch were found on Oracle bones. The Animal names of Stem-Branch were applied into the Month system around 206 B.C. (between the 秦 Chin and 漢 Han dynasties). The Stem-Branch was applied into the Year system around 164 B.C.
Almost every Chinese dynasty had its own calendar. One reason was that the new emperor needed to announce the new rules including the new calendar to the public. The other reason was that the astronomer's officers had to make some adjustments on the calendar in order to match the sun and moon in the sky.
The first day of a year has been changed many times in Chinese calendars. The current Chinese calendar uses the first day of the first month of Tiger month as the first day of a year, which was initially adopted by the 夏 Shiah dynasty (2183-1752 B.C.). Today, some people consider the Winter Solstice as the first day of a year in the Chinese Fortune-Telling System, because that makes more sense from the astronomy view.
Leap Month Problems in the Chinese Lunar Calendar
The Chinese Lunar Calendar system is not perfect, because the length of a solar month is not always greater than the length of a lunar month. As a result, a lunar month might contain two solar Center Points. Those months are found in the winter of 1548, 1680, 1700, 1719, 2053, 2072, and so on. This imperfection causes trouble in the assignments of the Chinese Lunar Leap Month. That's why there is no simple formula for Chinese Lunar Calendar conversion.
Because of this imperfection, we need to add a new rule to adjust the Chinese Calendar system.
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By Master Allen Tsai on February 4, 2023