Chinese calendar is not just lunar calendar

I was in Beijing last week. It was cold, and the first thing people talked about in any casual dialogue was the forthcoming jie qi (節氣) which seemed to offer its miraculous prediction about the weather to be expected. Da han (大寒, literally “severe cold") was the jie qi to arrive soon as stated in our traditional Chinese calendar. Coincidentally, Hong Kong Observatory here also forecast a drastic fall in temperature the coming weekend. "Severe cold" is among the 24 so-called jie qi (節氣) which tells roughly the kind of weather to be expected throughout the year. Contrary to my presumption that most people are aware of the relationship between the solar cycle and weather, most Chinese people have taken for granted the 24 jie qi as being a feature of the Chinese lunar calendar, and when talking about jie qi, we always refer to the Chinese lunar calendar. But the lunar phase is related to the moon's orbital cycle, and in theory gives little information on weather or season. The 24 jie qi shown on our lunar calendar in fact follow closely the solar cycle, and technically speaking they correspond to a set of exact values of the solar longitude. Our Chinese calendar is therefore a combined solar and lunar calendar.

Our "month" (literally "moon" (月) in Chinese) is defined by the duration between two successive full moons, normally around 28 to 30 days. We fix the 15th day of each month as the day on which a full moon is observed. The solar cycle is approximately 365¼ days. If a year is roughly a solar cycle, then a year may have 12 or 13 (for leap years) full moons, which add up to 354 or 383 days. The leap year, like in the Gregorian calendar, makes up for the cumulated missing days of a solar cycle after a number of (typically 3) 12-lunar-month years have elapsed. The naming of the year is another feature of the Chinese calendar. We have specifically two sets of naming elements, namely tian gan (“heavenly stems" 天干) and di zhi ("earthly branches" 地支). Tian gan has 10 elements (甲、乙、丙、丁、 戊、己、庚、辛、壬、癸) and di zhi has 12 (子、丑、寅、卯 、辰、巳、午、未、申、酉、戌、亥) which also define the 12 animal zodiacs. Each year is named according to an ordered pair formed by taking one element from each of tian gan and di zhi, for example, 「乙未」for this year and then「丙申」for the coming year. There are thus exactly 60 combinations of ordered pairs giving a cycle of 60 years that will nearly synchronize both solar and lunar cycles. Two consecutive years of 甲子 are thus 60 years apart.

Coming back to the topic of jie qi (節氣), which tells about seasonal changes, the fact that the lunar calendar defines the 24 jie qi on 24 specific days every year is a matter of presentation. If you take a close look at the dates of the jie qi* for the past 10 years, you will be surprised (if you don't know it yet) that they always fall on the same days of the Gregorian calendar, giving or taking a couple of days. For example, winter solstice (冬至) is always on December 22 or 23, which is actually the shortest day of the year (observed from the Northern hemisphere) and when the sun's longitude shifts to the southmost position, technically 270^o. In fact, the 24 jie qi always fall on the 4-8th day and the 20-24th of each Gregorian calendar month. "Severe cold" (大寒) is always around January 20, and soon after that "spring commences" (立春) in early February. The 24 jie qi correspond to fixed solar longitudes of 0^o, 15^o, 30^o, 45^o, ..., 285^o, at increments of 15^o, beginning with "vernal equinox" (春分) which has a solar longitude of 0^o and hence an equal day-night proportion. Further, except for "vernal equinox" (春分) and "autumnal equinox" (秋分) whose solar longitudinal positions are exactly at the equator and hence of particular importance, Chinese calendar also attaches a specific seasonal indicative term to every other jie qi, e.g., "moderate cold" (小寒), "severe cold" (大寒), "spring showers" (雨水), "insects waken" (驚蟄), "moderate heat" (小暑), "frost" (霜降), etc. The 24 jie qi are clearly just 24 solar terms and should be quite invariant on the Gregorian calendar.

There are many interesting and useful features of the Chinese calendar that are less known nowadays. The rules for determining a leap year and the choice of the month to repeat in that leap year have some interesting technical relevance to the presence of the first sun's principal term (at 30^o or its multiples of sun's longitude) in a month and hence the convenience for informing about seasonal changes. None of these are provided in the Gregorian calendar, but yet it is the calendar the world has chosen to use.

January 2016