Back to the
Ask An Astronomer page.
On Mon, 24 Apr 2000, Joseph wrote:
> I can't seem to reconcile the Julian day 2,451,545 for January 1, 2000
> with the year, 6712 since the beginning in January 1, 4713 BC, and the
> mean solar year of 365.2425, or anything close to that.
> It seems to be 36 days off. I accounted for leap years, leap centuries,
> etc.
This is because of three reasons. First, in the gregorian calender, every
year divisible exactly by 400 _is_ a leap year, like 2000 was. So, the
year 1600 was a leap year. This reduces the discrepancy to 35 days.
Secondly, before 1582, centuries were considered leap years, adding 45
days to the calender. This now means there are 10 too many days. The
final resolution comes from 1582, when the pope declared that the day
following October 4 was October 15.
What caused this craziness? The Romans had instituted the Julian
calender, with a leap year every 4 years. This makes the mean tropical
year to be 365.25 days. But, as you pointed out, it is slightly less than
this - 365.2425 days (plus or minus a few ten thousandths of a day). So,
every four hundred years the calender would get three days ahead. By
1582, the discrepancy was 10 days, meaning that the seasons were starting
to get seriously out of step with the calender.
Scientists of the time determined that the problem was with the length of
the year, and the Gregorian Calender was instituted, by order of Pope
Gregory XIII on October 15, 1582 (the day following October 4).
Still, not everybody went along. England didn't adopt the new calender
until 1752, and Russia waited until the communist revolution. Greece and
Turkey waited until the 1920s. Chaos reigned due to questions about rent
and interest charges for the shortened month. Different countries had
different dates, and even years, on the same date. Eventually, however,
the Earth's rotation prevailed, and everybody adopted the new calender.
The Gregorian Calender is accurate to within a day every 1600 years.
Thanks for asking - it was a good question!
Sincerely,
Kurtis Williams
Back to the
Ask An Astronomer page.