Although the most accurate calendar to that date, the Julian calendar was
not perfect. The length of a solar year is not actually 365.25 days, it is
365.242 days. The difference is small, but over the years this would add up.
Within a thousand years the calendar would be 7 days off the proper solar
reckoning.
This bothered Pope Gregory VIII. The central celebration of Christianity is
Easter, the day when Jesus rose from the dead. In 325 at the Council of Nicea
it had been determined that the date of the celebration of Easter would be
based upon the vernal equinox.
However, with the days shifting, the vernal equinox was moving further and
further away from the time when Easter should be celebrated. The vernal equinox
was by Gregory’s time over a week before the celebration, if not more.
This was a problem, and a reform was needed. The leap year system would
remain intact, however 3 leap years out of every 400 years would be skipped.
Every centennial year ending in 00 except for ones divisible by 400. Thus 2000
was a leap year, although 2100 will not be.
Like Julius Caesar, Pope Gregory VIII had to correct the calendar as well.
He did this by skipping over 10 days in October in the year 1582. All of Europe
went along with this change except England.
England had
just recently split with the Papacy during the reign of Henry VIII, and it was
decided that England
would remain on the old calendar. Thus the dates in England
were always 10 days different than those in the rest of Europe
until the finally went along with the reform themselves in 1751.
It is this calendar, developed by Pope Gregory VIII, which we still follow
today. It is the most accurate calendar in history, although it still has its
inaccuracies. Firstly, our calendar years still do not match up perfectly with
the revolution around the Sun. However, it will make no significant difference
for thousands of years, not even of a day.
Part of the problem is that the Earth itself is slowing down in its
rotation. Every day that passes is .00000002 seconds longer than the day
previous. While this is not a significant difference, it does add up, and could
cause calendrical problems further down the line. However this is something
that no one will need to worry about for thousands of years.