The Gregorian calendar year 2024 is 12 days longer than the lunar calendar year of Jiachen. Why?

The Gregorian calendar year 2024 is a leap year, with 29 days in February and 366 days in the whole year, while the upcoming lunar year of Jiachen (Year of the Dragon) is a common year with 354 days in the whole year. The Gregorian calendar year 2024 is 12 days longer than the lunar year of Jiachen. What's going on?

Yang Jing, a member of the Chinese Astronomical Society and director of the Tianjin Astronomical Society, explained that the Gregorian calendar, commonly known as the "solar calendar" in my country, is based on the movement cycle of the earth's revolution around the sun, which is a tropical year. The length of a tropical year is 365.2422 days, or 365 days, 5 hours, 48 ​​minutes and 46 seconds.

Since the tropical year is not an integer, the Gregorian calendar stipulates that a leap year is set every 4 years. Any year that is divisible by 4 is a leap year, with 29 days in February and 366 days in a year; any year that is not divisible by 4 is a common year, with 28 days in February and 365 days in a year. 2024 is divisible by 4, so it is a leap year. However, if it is a whole century year (such as 1800, 1900, 2100) that is divisible by 100 but not by 400, it is still a common year.

The lunar calendar is my country's traditional calendar. Its arrangement of years, months and solar terms is based on two natural cycles: the waxing and waning of the moon and the annual apparent motion of the sun.

The lunar calendar determines the month according to the changes in the moon's new and full phases. There are 12 months in a year, with a long month of 30 days and a short month of 29 days, totaling 354 or 355 days in a year, which is 10 to 12 days less than the Gregorian year (also called the tropical year) of 365 or 366 days.

In order to adjust the time difference between 12 lunar months and a tropical year, the ancient Chinese adopted the method of setting "leap months". The current method of "setting leap months" in the lunar calendar is "seven leap months in 19 years", that is, adding one month every 2 to 3 years. The added month is called a "leap month". The year with a leap month is called a leap year, which contains 13 lunar months and is 384 or 385 days long. On the contrary, the year without a leap month is called a normal year.

Yang Jing said that the rules for "intercalary months" are based on the 24 solar terms. The lunar calendar uses 12 solar terms as the symbols of the 12 months, that is, each month has a solar term. If there is no solar term in a lunar month, this month is called the "leap month" of the previous month.

The upcoming lunar year of Jiachen has no leap month, so it is an ordinary year. Its corresponding Gregorian calendar dates are February 10, 2024 to January 28, 2025, a total of 354 days.

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Source: Xinhua Viewpoint Official Weibo

Editor: Yang Yaping