Re: Revenge of the Y2K Bug?

[dempson@actrix.gen.nz (David Empson)]

Doug Browning <wpg750@mb.imag.net> wrote:

> There was no change. The Gregorian Calendar was designed that way. There
> are 365.25 days/year

Sorry, that is not correct.  The Julian Calendar assumed there were
365.25 days per year, which is slightly too long.

According to my "Pears Cyclopaedia", the mean duration of a solar year
is approximiately 365 days, 5 hours, 48 minutes and 46 seconds, i.e. 11
minutes and 14 seconds less than 365.25 days.

Going at 365.25 days per year, the cumulative error after 400 years is
74 hours, 53 minutes and 20 seconds, which is a couple of hours over 3
days.  These three days must be removed to keep the calendar in sync.
The Gregorian Calendar requires that three out of every four centuries
not be a leap year, thus removing the extra three days.  Only centuries
that are multiples of 400 are leap years.  This rule continues
indefinitely - 2100, 2200 and 2300 will not be leap years, 2400 will be,
2500, 2600 and 2700 will not be, etc.

A further correction would eventually be needed to allow for the extra
two hours, e.g. by removing the multiple-of-400 leap year about once
every 4000 years.  Even this isn't quite right and higher magnitude
corrections would eventually be needed, but by the time we get to this
scale other factors are likely to start interfering (e.g. gradual
slowing of the Earth's rotation due to tidal pull by the Moon).  I think
we can leave this for our descendants to worry about.

> so that means 365 days in a regular and 366 in a leap year (every four
> years), except for 1700, 1800, and 1900 as required to bring the calendar
> in line with the solar year.

The correction to bring the calendar in line with the solar year was
done when the Gregorian Calendar was introduced: in 1582 for Europe,
1752 for England, and 1918 for Russia.  In each case, the appropriate
number of days was deleted to bring the calendar back in sync.  11 days
had to be deleted when England changed over, which sounds about right
(three days for every 400 years since Julius Caesar's time).

1700, 1800 and 1900 were simply following the ongoing correction
requirements of the Gregorian Calendar.

-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P.O. Box 27-103, Wellington, New Zealand