How does the Gregorian calendar differ from the Julian calendar?
The Julian calendar had a simple rule of intercalation adding 1 day to every fourth year (February was originally the last month of the year which is why it has the least days and got the leap day). This created a mean year length of 365.25 days. However, this was not terribly accurate. One Pope Gregory XIII decreed the use of a new calendar in the 16th century. This calendar was almost exactly the same as the Julian Calendar but it dropped the intercalary day on every 100th year, while keeping it on every 400th year. This created a 400 year cycle, with 97 leap days for every 400 years, creating a mean year length of 365.2425 days. This closely matches up with the vernal equinox year (which is the length of time it takes between passages the sun on the vernal point on the celestial sphere) which is 365.242374 days, leaving a difference of only 0.000126 days (about 10.9 seconds) per year. This would add up to a day after just under 8,000 years, which could be easily removed by skipping
