384 days in a year! Why?

2025 is 19 days shorter than the Year of the Snake

The New Year's bells are ringing, and the first day of 2025 has arrived. Careful members of the public may find that the Gregorian calendar year 2025 has 365 days, which is shorter than the upcoming lunar year of the Snake, which has 384 days. And the 2024 year has 366 days, which is longer than the 354 days of the Dragon Year. Why is the "time difference" between the Gregorian calendar year sometimes longer and sometimes shorter?

"The difference between the Gregorian calendar and the lunar calendar is related to the rules for intercalation." Wang Kechao, director of science popularization at the Purple Mountain Observatory of the Chinese Academy of Sciences, explained that the current Gregorian calendar is a solar calendar. The time interval between two consecutive passes of the sun through the vernal equinox is a tropical year, with an average length of about 365.2422 days. Since the average length of a tropical year is not an integer, the Gregorian calendar has ordinary years and leap years, with 365 days in an ordinary year and 366 days in a leap year. In the Gregorian calendar, every year that is divisible by 4 is a leap year, with an extra day, February 29, but a century year must be divisible by 400 to be a leap year, following the rule of "a leap year every four years, no leap year every 100 years, and a leap year every 400 years."

my country's traditional lunar calendar takes into account the cycle of the moon's waxing and waning and the length of the tropical year. It is a yin-yang calendar. Wang Kechao said that a synodic month is about 29.53 days on average. Rounding off, the lunar month is divided into a long month and a short month, with a long month of 30 days and a short month of 29 days. A lunar year has 12 months and a length of 354 or 355 days, which is about 11 days shorter than a tropical year.

In order to make the length of the lunar year close to the tropical year and the order of the months consistent with the four seasons, the lunar calendar uses the method of setting leap months to make up for the "time difference". The arrangement of leap months is related to the solar terms. In the lunar year, the first synodic month without a middle solar term after the winter solstice is determined as a leap month. Middle solar terms refer to the 12 solar terms in the even-numbered sequence of the 24 solar terms arranged from the beginning of spring (such as Rain Water, Spring Equinox, Grain Rain, etc.). A leap year in the lunar calendar has 13 months and is about 384 days long. There are about 7 leap years in every 19 lunar years, so there is a saying that "there are seven leap years in 19 years".

The Gregorian calendar year 2024 is a leap year with 366 days, while the lunar calendar year of Jiachen Dragon is a common year with 354 days. This year's Gregorian calendar year is 12 days longer than the lunar calendar year. The Gregorian calendar year 2025 is a common year with 365 days, while the lunar calendar year of Yisi Snake is a leap year with a leap June, with 384 days, which is 19 days longer than the Gregorian calendar year.

The calendar is not only a rule for setting time, but also a carrier of cultural heritage. "The Chinese created a traditional calendar system based on the laws of the movement of the sun and the moon, the cycle of phenological changes, and the rhythm of agricultural production and life. It contains the wisdom of following the time and respecting nature, and also shows vitality in adapting to the changes of the times," said Wang Kechao.

Source: Xinhua News Agency

Reporter: Wang Jueyin, Qiu Bingqing