Thus it was written in the epistle of Costas Anastassiades,
>
> This is fascinating and complicated stuff. I've just checked with my
> Windows95 calendar and it reports that February in the year 2000 has 29
> days, so it's final. Year 2000 is a leap year ;))))
>            Now I'm no mathematician but I can't think of a year (at least
> in the future) that's divisible by 400 and not divisible by 4, so can we do
> away with the first rule ("divisible by 400")? That would leave "evenly
> divisible by 4 AND not evenly divisible by 100". The next such year is 2100
> (according to my truly poor math skills), that's 104 years from now. So
> won't "divisible by four" do for now or is an exact calculation of leap
> years necessary as we are actually producing systems that have an expected
> life span of more than 100 years ? or perhaps our software calculations
> take into account the next two centuries ? or perhaps this list only needs
> a spark.....:)
>
> Costas
>

Costas,
  Perchance this will clear things up a little (or it may make them worse :-).
The extra day added into February on leap years is a compensation that we make
for the solar system.  There is a discrepancy between the speed that the earth
rotates and the speed it goes around the sun.  The earth takes 365 and almost
one quarter days in its yearly cycle.  Thus, every four years we must
compensate by throwing those "quarter days" together in one day and have a
366 day year.  However, it isn't quite a full quarter day.  So every 100 years
we discover that we have overcompensated by about a day and we skip leap year
on the hundreth year to make up for this.  With a little more precision, we
discover that that decision actually overcompensates in the other direction by
the amount of about one day in 400 years.  So to make up for that lack, every
fourhundredth year we go ahead and have leap year so as to adjust back in the
other direction.  I suspect that even this doesn't come out exactly and that
there is probably a rule about what to do every 1000 years or 4000 years but
that you just have to give up sometime and take what comes.  Thus, as has been
said before, leap year is every year which is divisible by 4 excluding the
years which are divisible by 100 *but including those years which are divisible
by 400*.

Hoping this helps,
Ted
--
Ted Ashton ([log in to unmask]) | From the Tom Swifty collection:
Southern Adventist University    | "My pencil is dull", said Tom pointlessly.
Collegedale, TN  37315           |
(423) 238-2703                   |