# Chinese Calendar Math

## Chinese Calendar Math

Happy Chap Goh Mei, or 元宵节快乐! Today is the 15th day of Chinese New Year, and as we celebrate today, let’s take a read at the math behind the Chinese calendar – how it is actually calculated.

## A little background

The Chinese calendar’s origins can be traced as far back as the 14th century BCE. The Shang oracle bones give evidence of a lunisolar calendar which has been much modified but persists to this day. It is believed that the Emperor Huangdi introduced the calendar between 3000 and 2600 BCE, or around 2637 BCE. It is also thought that his minister Ta Nao prepared the 1st calendar, called the Chia-tzu or Kan-chih system, which is translated as “the system of cyclical characters”.

The Chinese calendar is lunisolar. It is based on exact astronomical observations of the sun’s longitude and the Moon’s phases. It attempts to have its years coincide with the tropical year and shares some similarities with the Jewish calendar.

Although the Chinese calendar originated in China, these days, the Gregorian calendar is used for civil purposes. However, the Chinese calendar is still observed among various Chinese communities around the world. It is used to determine festival dates, such as Chinese New Year, as well as auspicious dates, such as wedding dates. It is also used to determine Moon phases because it follows the moon.

## Calculating Chinese New Year

According to Helmer Aslaksen of the National University of Singapore, there are 2 rules of thumb used to calculate the new year in the Chinese calendar. The 1st rule of thumb is that Chinese New Year should be the new moon closest to the beginning of spring (in the northern hemisphere), known as Lìchūn. This rule is correct most of the time, but it can fail if Lìchūn falls close to halfway between 2 new moons. It failed in 1985 and will fail again in 2015. Chinese New Year will always fall between January 21 and February 21.

The 2nd rule of thumb is that most of the time Chinese New Year will fall 11 (or sometimes 10 or 12) days earlier than the previous year, but if that means that the event would be outside of the Chinese New Year range of January 21 to February 21, a leap month must be added so Chinese New Year jumps 19 (or sometimes 18) days later.

Never knew that there is math involved in calculating the days to the next Chinese New Year!