Understanding the Base Year and Discrepancy Between the Gregorian and Julian Calendars

The Gregorian calendar has been the internationally recognized civil calendar since 1582, but it's a common misconception that this calendar ever shared its days with the Julian calendar. This article explores the concept of the base year and the discrepancies that existed between the two systems.

The Base Year of the Gregorian Calendar

The base year for the Gregorian calendar is 1582, the year in which it was introduced. On October 4, 1582, the date changed to October 15, 1582, marking the official end of the Julian calendar. This transition was strictly forward-looking, as the Gregorian calendar was designed to correct the drift in the vernal equinox that had accumulated over centuries under the Julian calendar. The base year 1582 remains the starting point for the Gregorian calendar, and no year prior to this is part of its chronology.

The Implementation and Impact of the Gregorian Calendar

The Gregorian calendar was first implemented in 1582 by Pope Gregory XIII, based on papal bull Inter gravissimas. This document outlined the corrective measures needed to revise the calendar, ensuring that future dates would align more accurately with astronomical phenomena. The key changes included the rules for leap years, which were intended to ensure that the spring equinox would occur around March 21, the date traditionally used for calculating Easter.

Inter gravissimas stated:
‘

Next, so that the equinox will no longer recede in future from the twelfth day before the Kalends of April [i.e. 21 March] we decree:

that the bissextile day every fourth year shall continue as the custom is now except in centurial years although these were always bissextiles before and we wish the year 1600 to be bissextile as well. After that, however, the centurial years that follow shall not all be bissextiles, only every fourth centurial year shall be bissextile, thus the years 1700, 1800, and 1900 shall not be bissextile. But in the year 2000 the bissextile day shall be added in the usual way with February containing 29 days, and then the same order of leaving out and adding the bissextile day shall be observed in each period of 400 years ever after.

This new set of rules significantly reduced the deviation between the calendar and the solar year, tightening the alignment and ensuring that future calendar dates would remain accurate.

Parallel Coexistence of Both Calendars

While the Gregorian calendar was officially adopted in 1582, the Julian calendar continued to be used in some regions for a considerable period after. This parallel use is evident in historical records, such as the case of George Washington, who had his birthday listed as February 11, 1731, under the Old Style Julian calendar. When the New Style Gregorian calendar was adopted in 1752, his birthday changed to February 22, 1732. However, this is a unique historical scenario and not a common practice in modern times.

The Discrepancy Between the Calendars

The fundamental reason for the discrepancy between the Gregorian and Julian calendars lies in their leap year rules. Over the centuries, these differences led to the Julian calendar falling out of sync with the seasons. The Gregorian leap year rules corrected this drift, ensuring that the calendar year and astronomical year remained in closer alignment.

By the time the UK and its colonies adopted the Gregorian calendar, the discrepancy was 11 days. The transition was not instantaneous, as it took decades for different countries to adopt the new calendar, leading to various dates and discrepancies in historical documents for the years immediately following the implementation.

Conclusion

The base year for the Gregorian calendar is 1582, and it was designed to correct the discrepancies introduced by the Julian calendar over time. While there was a period of parallel use, the two calendars did not share any dates or days. Understanding this historical context helps clarify the evolution and accuracy of the calendar we use today.