The Gregorian calendar or the new calendar is the most used calendar in the world. As a modification of the Julian calendar, it was first proposed by the Calabrian doctor Alojzije Lili, and proclaimed in 1582 by Pope Gregory XIII, after whom it was named, through the papal bull Inter gravissimas. His years are counted from the year of the birth of Jesus Christ.
The Gregorian calendar was introduced because the average year in the Julian calendar was slightly longer than the tropical year, causing the vernal equinox to slowly go backwards in the calendar year, as well as the lunar calendar used to determine the date of Easter.
The Gregorian calendar solved these problems by eliminating a certain number of days, so that the calendar returned to synchronization with the seasons, ie the tropical year, and a small shortening of the average number of days in a calendar year, eliminating three Julian leap years every 400 years.
On the advice of the German astronomer Christopher Clavius (1538-1612) and the Neapolitan physicist and astronomer Aloysius Lilius (1520-1576), on February 24, 1582, Pope Gregory XIII (1502-1585) announced the reform of the existing Julian calendar with the papal bull Inter Gravissimas named after the first two words of her text. The papal bull contained the following provisions:
10 days will be left out of the calendar, so that after Thursday, October 4, 1582, Friday, October 15 will follow.
Each year is divisible by 4, except for the divisible by 100, where only those divisible by 400 are delinquent.
The leap year has one day more than the leap year, which is set at the end of February.
Easter will henceforth be calculated according to a new rule, tied to the new calendar.
The first day of the year will be January 1. This new calendar is named after Pope Gregory the Gregorian. According to him, the average length of the year was reduced to 365.2425 days (365 + 97/400 365.2425 because there are 97 leap years for every 400) which gives an error of 365.2425 - 365.2421890 0.00031 days ≈ 26 seconds. This means that this error will accumulate in one day for 1 / 0.00031 ≈ 3225 years, or somewhat shorter because over time the duration of the solar year decreases and the error increases.
The difference between the Gregorian and Julian calendars is that the Gregorian calendar has 97 leap years in every 400, and the Julian calendar 100. The Gregorian calendar has more