25 days, therefore, taking 365 days as the basic age and adding one more day every four years, we can get a good approximation. Julian calendar method has been used until16th century.
The difference between the approximate length and the actual length of that year accumulated about 10 days. In order to eliminate this difference, Pope Gregory XIII set the second day of julian calendar 1582 10/0/October 4th as 15, minus 10 days, which made the calendar correspond to the tropic year again and began to use the new Gregorian calendar.
As we all know, the common method of chronology in the world is the method of AD chronology, whose predecessor was julian calendar and Julius Caesar in Rome. For a long time, Rome was the center of western civilization, but there was no complete calendar. French scholar Voltaire once sarcastically said: "The Romans won many battles, but they didn't know when." After Caesar proclaimed himself emperor, he was determined to change this situation. In 46 BC, Caesar invited the Greek mathematician and astronomer Sotchini to Rome to sort out and compile the calendar.
In the end, Caesar adopted Sotchini's suggestion and re-edited the calendar according to the Egyptian solar calendar, that is, julian calendar. In 45 BC 1 year 1 month1year, Caesar ordered the implementation of this calendar in Rome, which improved the previous chaotic situation.
The first, second and third years of the calendar are all 365 days, and the fourth year is 366 days, which is a leap year. This cycle of 365, 365, 365, 366 is an integer every year. However, this kind of calendar also has problems.