What is the difference between the Gregorian and Julian Calendar, and why is it important to genealogy?

Today, we take for granted our 365-day year and the celebration of spring -- the vernal equinox -- on 21 March of every year, but it has not always been so. There was a time when the calendar was "off" by a little bit each year, which caused the the calendar date of the vernal equinox to fluctuate, an issue for the Roman Catholic Church in celebrating Easter. By the time a correction was made to steady the date, the date of the vernal equinox had lost 10 days, arriving on 11th of March rather than the 21st. The key in correcting the time came by modifying the cycle of leap years. This modification in the calendar, authorized by Pope Gregory XIII in 1592, became known as the Gregorian Calendar -- the calendar we use today -- replacing the Julian Calendar introduced by Roman Emperor, Julius Caesar. However, not everyone subscribed to the change, and in some countries the Julian Calendar was used into the 20th Century. Over time, however, other countries began adopting the calendar, but even today it is not universal.

The Julian Calendar, in effect from 45 BC, divided the 365 days per year into 12 months, adding a Leap Year, every four years -- that day we celebrate every four years on February 29. The problem, however, was that this calendar did not coincide exactly with the solar year, and changes over time resulted in a fluctuation of the accepted dates for the seasonal equinox; thus, the adoption of the Gregorian Calendar.

The Julian Calendar, then, was in use from 45 BC to 1592 AD. The Gregorian Calendar has been in use since 1592, but not in all countries. To better understand dates found in records, it is important for researchers to know when countries changed from the Julian to the Gregorian Calendar. England, for example, did not adopt the calendar until 1752, which has implications for those researching ancestors during the American Colonial Period. Prior to 1752, the new year began on March 25th. In some New England records you may find two dates separated by a slash 1701/02, taking into account these calendar differences.

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