Astronomical and Civil Calendar

Like the West, Chinese astronomers had discovered the average time for the sun, as observed from the earth, to return to about the same position in its path in 365.25 days, and that within this period of time, the moon had been changing from one new moon to the next for slightly more than 12 times, averaging a cycle of 29.5 days, and hence they found the equation to bring Li-chun (立春 or Begining of Spring) close enough to the 1st day of the 1st lunar month of the year, i.e. 365.25 x 19 = (12 x 19 +7) x days/month, which means 7 intercalations in 19 year. The significance of which is not to confuse the farmer because Li-chun will simply be slipping away if there isn’t any intercalation.

Li-chun as part of 24 Solar Segments (二十四節氣 Er-shi-si Jie-qi), is one of the 24 specific positions in the ecliptic. By taking Chun-fen (春分), or Vernal Equinox at 0 degree longitude as defined by the West, Li-chun is then 315 degrees in longitude. Chinese people accustom to Li-chun as the beginning of the year. It is generally referred as Civil Calendar. However, it is not the way the astronomers use for calendar calculation.

Chinese astronomers employ the concept of Yin and Yang in calendar calculation and believe Yang starts to grow right after the most Yin and vice versa. By putting the 12 Earthly Branches around a Tai-ji Yin-Yang Diagram (太極陰陽圖), with Zi (子) as the leading Branch, then Yang will start to grow at Zi while Yin will start to grow at Wu (午). To put in seasonal context, both Winter Solstice (冬至 Dong-zhi) and midnight will then signify the beginning of Yang while both Summer Solstice (夏至 Xia-zhi) and mid-day will then be the beginning of Yin.

Together with Sexagenary Cycle as the recording system, the calendar is best calculated in the year of Jia-zi (甲子), the month of Jia-zi, the day of Jia-zi and the 辰 (Chen or 2-hour period) of Jia-zi. Winter Solstice is the second segment in the month of Zi and the Chinese called it Zi-zheng (子正) which means the middle of the month of Zi. Likewise, 12 o’clock midnight will be Zi-zheng as well.

Wait a minute, are you suggesting there is month of Jia-zi in the year of Jia-zi, but that’s not what people said. People said the year of Jia-zi starts with the month of Bing-yin (丙寅), there should not be any month of Jia-zi.

Yes and no, as this is the most confusing part about Chinese Calendar. The calendar we were told is in fact a civil calendar, while the astronomers are calculated by using Winter Solstice (the most Yin) as a reference point. Therefore, the year of Jia-zi in fact begins in the Winter Solstice of the previous year Gui-hai (癸亥) till the coming Winter Solstice in the year of Jia-zi. There is month Jia-zi in the year of Gui-hai.

However, this is only the relationship between the Solar system and the recording system, how does the lunar system come in the picture?

Let’s imagine if the above said date and time happens to be a new moon, then from one of this phenomenon to the next is a kind of a common cycle for sun, moon, and the sexagenary cycle. By using Chinese Remainder Theorem, we know there will be solutions in multiple. If it is required to consider the alignment of planets as well, then the length of the cycle could be astronomical. We don’t have the history to witness the entire cycle, but could only imagine there exists one. By then, this cycle is called the Initial Epoch. Every calendar before Tang Dynasty had different Initial Epoch due to different parameters were being used, but they were all checked against the recording system in order to make sure continuity.

Remember, the equation holds only if we are talking about average days per year and month, or Mean Sun and Mean Moon. Ever since Tang Dynasty, the Chinese calendars were changed from Mean Sun - Mean Moon to Mean Sun - True Moon and further changed to True Sun - True Moon at the beginning of 1912CE. Without Mean Sun and Mean Moon, there is no way we can calculate the Initial Epoch.

Now, if the recording system was really first introduced in 2637BCE (since it was said that Huang-di took the power some time in 2697BCE), then in fact they were calculated by using the previous Winter Solstice as reference point. It was definitely clear that it did not start in February, be it proleptic Gregorian nor Julian.