Many of us must have noticed that often the dates of birth or death of famous personalities that occurred before 1918 are indicated in double format: in the old and new style. This discrepancy is caused by the adoption of the Gregorian calendar. Let’s figure out what it is.
Time is a rather ambiguous system that requires constant correction. So, for example, even before our era in Ancient Rome, a leap year was invented, which, by introducing an additional day in the year, evened out the difference between the accepted time and the astronomical one. The astronomical year according to the Roman calendar is 365 days and 6 hours. Thus, a correction was required every 4 years. This calendar was called Julian and acted everywhere.
Another concept that you need to know to understand the need for the introduction of the Gregorian calendar is the tropical year. This is the name of the time between the two equinoxes. The tropical year was 365 days, 5 hours and 49 minutes, that is, 11 minutes “behind” the Julian calendar. It would seem that the difference is small, but every 128 years it increased by a day and once reached 11 days. The vernal equinox also shifted and once began to seriously affect the date of the celebration of Easter. When the Christian and Jewish Easters separated from each other by a critical time distance, Pope Gregory XIII intervened.
The head of the Catholic Church in 1582 made a strong-willed decision - to change the calendar. After October 4, in 1582, October 15 came immediately in Portugal, Spain, Italy, Poland, and Lithuania. The transition from one calendar to another lasted almost 5 centuries.