1582 Why is there no October?
As we know, the period of the earth's revolution around the sun is 365.2422 days, but in the earlier julian calendar, because every four years, 1 leap day is added, that is, February 29th, the average annual average is 365.25 days. Don't underestimate the difference of 0.0078 days (about 1 1 minute 14 seconds). It doesn't matter for a year or two, or even ten or twenty years, but it will be serious after hundreds of years of accumulation. In fact, in the 13rd century, the Julian year was 8 days longer than the actual astronomical phenomena, and the vernal equinox actually became13 in March, while astronomy stipulated that March should be 2 1. Roger bacon was an outstanding thinker, scientist and philosopher at that time. He wrote a letter to Pope urban IV in 1263, urging them to study and solve this big problem (at that time, the right to make calendars was in the church).
But no one expected that this seemingly simple question stumped the Holy See, which dragged on for more than 300 years, leading to the further expansion of the error to 10 days. It was not until 1582 that Pope Gregory XIII agreed to an amateur astronomer's plan and issued a decree to change julian calendar into the Gregorian calendar. In fact, there are two main changes: the 10 day that came out more than one year was deducted, specifically the day after Thursday, 1582. In the future, all century years that are not divisible by 400, such as 1700, 1800 and 1900, are no longer leap years, and only those years that are divisible by 400, such as 1600 and 2000, are still leap years. This actually means an increase of 397 leap days every 400 years, three days less than the original. That is to say, in the new Gregorian calendar, the average length of a year is 365.2425 days, which is only 25.9 seconds longer than the actual age, which is enough to ensure that there will be no mistakes in two or three thousand years.