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Why Chinese Leap Year has 13 months (Leap Month)?


Chinese Luna calendar is based on the moon’s orbital cycle to the sun. 

A full orbital cycle for the moon to go around the sun is +/-29.5306 day.  It varies and depending on the length of the full moon and half moon of the month, therefore; some month will have 29 days and month will have 30 days. 

Generally, the day of the full moon is fixed on the15th day of the Luna month.

The orbital cycle of our Earth to the Sun is 365.2422 days – (We call it Solar Year.) If we calculate the Luna year as 12 months x 29.5306 days = 354.3672 days – This will equate as1 Luna year being 10.875 days shorter than 1 Solar year. 

In 3 Luna years, there will be about a total of 1 month of difference in days to 1 Solar year! This means the Luna New Year will drift further and further away from the Solar calendar year.

In order to control this problem, Chinese calendar scientists have introduced the Leap Year with 13 months (Leap Month) system. This is to set the Luna New Year to be roughly on the same foot step as the Solar year by adding1 extra month in every 3 years. 

Besides from the orbital cycle of the Moon to the Sun, Chinese calendar scientists have also created the 24 Ji and Qi’ (Season and Qi) for the Earth orbital cycle to the Sun.

Each of these Ji and Qi has its own unique term for identifying the changes of “season and weather” during the year. These terms are such as “Establishing of Spring”, “Rain Water”, “Wakening of Worms”…..”Bright and Clear”…”White Dew”…, etc. 

The Ji and Qi were mainly developed as the Farmers’ Calendar due to the agricultural nature of the country.

Based on the fact that each Solar year has 365.2422 days, therefore, there should be 365.2442/24=15.218425 days between each Ji and Qi. 

Under normal circumstance, each Luna month will carry 1 Ji and 1 Qi – unless it is a Leap year with 13 months (Leap Month), where there will only be either 1 Ji or 1 Qi for this special month.

How to determine which month is the Leap Month in a Leap Year? 

In a Leap Year – there will be a total of 13 months and 25 Ji and Qi.

Obviously, we can see that each of the 12 months will have their normal 1 Ji and 1 Qi= 2 in total.
Except, when an odd month that has only 1 Ji or Qi. This month will be used as the extra month (Leap Month) for the Leap Year.

For the fun of Chinese tradition customs:

Chinese consider the Leap Year which has two “Coming of Spring” seasons as “Super Auspicious” for marriage during the whole year. 

On the contrary, Chinese will not use the whole Leap Year that has no “Coming of Spring” for marriage and it is called the “Blind Year”! 

Lockerbie, Scotland
March 6th, 2012



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