Leap year refers to the year with leap day in Gregorian calendar or summer calendar, and the year with leap month in China lunar calendar. The period of the earth's orbit around the sun is 365 days, 5 hours, 48 minutes and 46 seconds (365.438+09 days), which is the tropic of cancer. The Gregorian calendar has an average of only 365 days a year, which is about 0.2422 days shorter than the tropical year. The remaining time is about one day in four years, plus February of the fourth year, making the calendar year of that year 366 days, and this year is a leap year. The current Gregorian calendar has 97 leap years every 400 years. The average summer calendar is only 354 days a year, which is 0.367 1 day shorter than the 12 months of the lunar calendar. In order to make the first day of each month coincide with the new moon, it is stipulated that the end of 1 1 year will increase by 1 day every 30 years. This calendar year has 355 days, which is a leap year. As a kind of China lunar calendar, the number of days in each month depends on the monthly deficit, and the time of one year is based on 12 months, and the normal year is about 1 1 day less than a tropical year. In order to close the earth's orbit around the sun, that is, the tropic year, one month is added every two to four years, and the added month is a leap month. The year with leap month is 13 months, and the calendar year is 384 or 385 days. This year is also called leap year.

According to a leap year every four years, the average annual calculation will be 0.0078 days, and it will increase by about 3 days after 400 years. So every 400 years, there will be three leap years. Therefore, it is stipulated that the Gregorian calendar year is a whole hundred, the leap year must be a multiple of 400, and the average year is not a multiple of 400.

Zheng XX, Class 4, Grade 5