Re: Ponte Alto Baronial Birthday

[schuldy@zariski.harvard.edu (Mark Schuldenfrei)]

Greetings from Tibor.

Corwyn wrote:
  The second one will be in 2000. 2000 is also a leap year.

Kendrick wrote:
  ***bzzzt***  Sorry, but thank you for playing.  The year 2000
  is not a leap year.  Every now and then they have to skip one
  to get things back on track.  (Anyone want to better explain it?)

Corwyn was right, actually.

It's been a while since I worried about this, but here is the algorithm.
(Note that Lotus 1-2-3 version 1 got this wrong.)

   Regular years are not leap years.
   Except: every fourth year IS a leap year.
   Except: every hundredth year IS NOT a leap year.
   Except: every four hundredth year IS a leap year.

Why?  Think about it this way.  The Earth spins around the sun, and should
return to the same relative spot every year: but it doesn't.  We measure
our days and years by how quickly the earth spins like a top, and the two
don't come out evenly.  Every 365 days and nearly a quarter, the planet
returns to the same relative place around the sun.

So, if every 4 years or so, we didn't slip an extra day in there, within a
century we'd be off 25 days, nearly.  Since the rotation around the sun is
a little less than a quarter day, every few years we don't insert the day,
but there are successive approximations.  In fact, the approximation above
is off even so, but the next adjustment will be a "once in a civilization"
sort of event, if I recall aright.  Something like a 10,000 year period.
(Forgive me, but that number is a guess:  I simply recall thinking that for
all purposes, none of us would ever care.)

Don't confuse this with leap seconds...  Those seconds correspond to the
spin around the earth's axis, which also isn't precisely 24 hours.  But the
second is very close indeed, and we slip a second or so every few years.  A
day we would notice, but a second or two?

There is also the issue that the Earth's rotation is a bit slow, and so
there are occasional adjustments of atomic regulated clocks, but those don't
really concern us either.  Yes, the rotational speed of the Earth around
it's axis is also slowing, and so is the rotation around the sun.   You
should live so long... (:-)

In period there were a number of attempts to make this come out even,
including a time post period when (If I recall) March 1st was made to
correspond to March 17th, in order to reclaim the springtime because of a
failure to have leap years.

It's no wonder the ancients were confused by time.  Precise timekeeping also
began to allow navigation, and brought an end to the Middle Ages, as much as
political considerations did.

Forgive me if I err, if you please.  It has been years since I studied the
topic, and it can be fascinating, if complex.

	Tibor