Why are there 365 days in a year? Why are there sometimes more days in a year and sometimes fewer days in a year? Who stipulates how many days there are in a year? Who is the calendar?

The earth revolves around the sun at the same time, which is called revolution. The period of the earth's revolution is a sidereal year, which is approximately equal to 10 second at 6: 09 on 365. Looking down from the north celestial pole, the direction of the earth's revolution is counterclockwise. According to the habit of sunrise in the east, it can also be said that it is from west to east. The orbit of the earth's revolution is an ellipse, and the sun is located at a focus of this ellipse, with a total length of 940 million kilometers. The average linear speed of the earth's revolution is about 30 kilometers per second. An important feature of the earth's revolution is that its axis of rotation is inclined to the orbital plane, and the included angle is equal to 66 33' (or the included angle between the equatorial plane and the orbital plane is 23 27', or the included angle between the ecliptic and the equator is 23 27'). In the course of the earth's revolution, the axis of rotation always points in the same direction, that is, the direction of the North Star. This is how the earth rotates on its side at the same time. But why are there 365 days and 366 days in a year? It takes 365 days, 5 hours, 48 minutes and 46 seconds for the earth to go around the sun. Because 365 days, 5 hours, 48 minutes and 46 seconds =365 days and 6 hours, in order to make up for the time difference, there are leap years every four years, and there are 366 days and 29 days in February. In the Gregorian calendar year, a year with leap days is called leap year, and generally a year is 365 days. Because the earth's cycle around the sun is 365 days, 5 hours, 48 minutes and 46 seconds (365,438+09 days), which is the tropic year, the Gregorian calendar sets a year as 365 days. The remaining time is about one day in four years, plus February, so the normal year is 365 days, February is 28 days, leap year is 366 days, and February is 29 days. So every 400 years, there are 97 leap years, with one day added at the end of February and 366 days in leap years. Calculation method of leap year: the number of years in the calendar year can be divisible by four, which is a leap year; Divided by 100 but not divided by 400 is an average year; Leap years can be divisible by 100 or 400. For example, 2000 is a leap year, and 1900 is not.