The Chinese calendar still used to determine holidays like the Chinese New Year is a bit different from the western calendar that we all follow. While the western calendar was invented by the Romans in 46BC, the Chinese calendar came to be used some times before 17th century BC!
As the Earth goes around the Sun, ~365 days pass for it to go all the way around once. Thus we have the familiar “year” as calculated by the western calendar. The year is broken up into twelve months for somewhat historical reasons, which as far as I’m concerned is pretty random.
The traditional Chinese calendar (also known as farming calendar 农历）incorporates both the lunar cycle, and the position of the sun. It’s a pretty accurate system for calculations made almost 4,000 years ago!
First let me explain how the Lunar Calendar works.
Instead of the Earth’s rotation around the Sun, the Lunar Calendar is based on the lunar cycle: 1 month = 1 cycle of the moon, each month starting on the day of the new moon. There are still 12 months in a year. Only now instead of a month varying from 29 to 31 days somewhat randomly, in the lunar calendar has 30 days in odd months and 29 days in even months. This comes out to 354 days in a year.
[The month is based on the fact that a lunar cycle is 29days 12hours 44min 3sec, almost right between 29 and 30 days. Then every 30 years, there’s 11 leap years with 355 days a year. Kind of confusing? Don’t worry about it.]
With the lunar calendar, you’ll always know the phase of the moon based on the day of the month. But since every year the lunar calendar is short by around 11 days, it’s possible to have the New Year in the winter, spring, summer, or fall.
Now for the traditional Chinese calendar
The Chinese calendar still bases the average length of a month on a lunar cycle. On top of that is added leap year calculations and what is called the “24 solar terms” (二十四节气), which is basically 24 equal periods in a year as calculated from the position of the sun. The result is a calendar year that has the same average duration as a solar year. (Again, kudos to the Xia Dynasty calendar makers back in the 17th century BC).
Like the lunar calendar, the traditional Chinese calendar also has either 29 or 30 days in a month, denoted as “small” or “big” month. Unlike the lunar calendar, the big and small months do not simply alternate in a year, their order is determined through calculations based on the sun’s position on the ecliptic. Every 100 years there’s 53 big months and 47 small months.
Each month starts with a new moon like with the lunar calendar (so you would know what day it is by looking at the night sky). But since the lunar calendar is missing 11 days every year, in 3 years it would be off by 33 days – pretty much a month. To make up for this, every 3 years there will be an extra month in the Chinese Calendar, with 13 months in a leap year. As to which month counts as the leap month in the leap year is determined by the solar term.
The history of the Chinese calendar goes back for thousands of years. There are ancient records of time keeping based on the sun, the moon and the stars. The calendar system is recorded through a system called the 10 heavenly stems and 12 earthly branches dating back to the Shang dynasty, ca. 1250 BC.
Back in early civilization when farming was an integral part of life, a “year” is the most important cycle of time to keep track of. It’s pretty incredible how much they gleaned from pure observation.