What is the definition of leap year?

The Gregorian calendar year is a multiple of 4, usually a leap year. However, the Gregorian calendar year is a whole hundred, and it must be a multiple of 400 to be considered as a leap year. "The general way to judge whether a year is a leap year is to divide the number of years in this year by 4 or 400. If the divided quotient is an integer with no remainder, then this year is a leap year; If there is a remainder, then this year is a normal year.

When the Gregorian calendar year is not an integer, only the last two digits of the year number are multiples of 4. If the last two digits of the year number are multiples of 4, this year is a leap year. When the Gregorian calendar year is an integer hundred, we only look at the thousands and hundreds of years. If these two digits are multiples of 4, then this year is a leap year, and vice versa.

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The origin of leap year

The usual explanation is that there are many days, hours and minutes in a year, so it is unnecessary to take the integer 365. After the cumulative number reaches 24 hours a day, the year with one more day is a leap year. This explanation just tells everyone how to calculate, which is artificially set.

The most fundamental reason is that the cycle of the earth's orbit around the sun is 365 days, 5 hours, 48 minutes and 46 seconds (365 days +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, and the remaining time is about one day every four years. So 1 day is added at the end of 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. According to a leap year every four years, it will count as 0.0078 days per year on average, so after 400 years, it will count as 3 days. So every 400 years, three leap years will be reduced. Therefore, the Gregorian calendar stipulates that when the year is a whole hundred, it must be a multiple of 400 to be a leap year; It is not a century year with multiples of 400, and even a multiple of 4 is not a leap year.

This is what is usually said: jump in four years, not jump in a hundred years, and jump again in four hundred years. For example, 2000 is a leap year, and 2 100 is a normal year.

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