Leap year is set up to make up for the time difference between the number of annual days and the actual revolution cycle of the earth caused by man-made calendar. The year in which the time difference is made up is leap year. Leap years have 366 days. So why isn't 2100 a leap year? Let's get to know it with me.
What about leap years?
Generally, the number of years in the Gregorian calendar is a multiple of 4, which is a leap year, otherwise it is a normal year;
However, the Gregorian calendar year is a hundred years. It must be a multiple of 400 to be a leap year. If it is not a multiple of 400, it is a normal year.
The years 2000 and 2100 are all hundreds of years.
2000 can be divided by 400. It's a leap year,
2100 cannot be divided by 400. It's a normal year.
Knowing the leap year algorithm, let's take a look at the detailed reasons
In the Gregorian calendar (Gregorian calendar), the year with leap day is called leap year. Generally, the year is 365 days and the leap year is 366 days.
Since the earth's cycle around the sun is 365 days, 5 hours, 48 minutes and 46 seconds (365.24219 days), that is, a year of regression, the Gregorian calendar sets a year as 365 days. The remaining time is about one day in four years, which is added to February, so it is 365 days a year in normal years and 28 days in February; Leap years have 366 days and February has 29 days.
In this way, 0.24219 days are counted less each year, so the remaining time is about one day accumulated in four years, which is added to February, but this is still not accurate enough, because 1-0.24219 is counted more × 4 = 0.03124 days,
Next, let's see that in 400 years, if there are 100 leap years, it will add 3.124 days. In this way, day and night will not be right. Therefore, we only give it 97 leap years in 400 years, so it will add about 0.124 days in 400 years (because this is only an approximate value), which is more accurate.
To sum up, if the time of 400 years is 365 days a year, it will be 400 less × 0.24219 = 96.876 days, so 97 leap years should be added. How to divide 97? Because of this, there is the current division method, one leap in four years and one leap in 400 years.
So there are 97 leap years in every 400 years.
Again, the calculation method of leap year: the number of years in AD can be divided by four, that is, leap year; A normal year is divided by 100 and cannot be divided by 400; Leap years are those divided by 100 or 400. For example, 2000 is a leap year, 1900 is not a leap year, and 2100 is not a leap year.