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Our calendar is just a mess.
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It's a very complicated mismatch
of different cycles
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and different lengths of time.
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And every once in a while, we
have to make an adjustment to
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those lengths of time to
make things match.
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And one way we do that is every
once in a while adding
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an extra day to our calendar.
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And that extra day
is February 29.
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When we have an extra day, we
call that year a leap year.
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So there are two fundamental
units of time that we use that
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are tied to actual,
physical events.
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One is the day, and that's the
time it takes for the Earth to
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make one rotation
around its axis.
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The other is the year, and
that's the time it takes the
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Earth to make one revolution in
its orbit around the Sun.
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Every other unit we use-- the
week, the second, the hour--
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is fairly arbitrary, but those
two are tied to actual,
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physical events.
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In a more well-behaved universe,
our calendar would
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be a lot easier to deal with
if 365 of these exactly
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matched one of those.
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But of course, that's not
the way it works.
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Actually, our year is made up
of about 365 and 1/4 days.
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That means that every time the
Earth had gone around the Sun
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once, it's actually rotated
365 and 1/4 times.
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If we rotate 365 and 1/4 times
every year, but we're only
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counting 365 of them in a year,
that means every four
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years, we've got an extra day
that hasn't appeared in our
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calendar, so we have
to put it back in.
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And that's where February
29 comes in.
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But that's not quite enough.
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The number of days
in a year is not
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exactly 365 and 6 hours.
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It's 365, 5 hours, 49 minutes,
and 16 seconds.
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That means if we do what I just
said, by the end of 100
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years, we've accumulated too
much time in our calendar, and
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now we need to take
a full day away.
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So every 100 years, on a year
that would've been a leap year
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going by the four-year rule, we
skip one, and we take away
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So every time the year is
divisible by 100-- that means
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1900, 2000, 2100--
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we skip the leap year.
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And so, on those years,
February has 28
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days and not 29 days.
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Except that's still
not quite right.
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There's an extra bit that we
need to correct again, because
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now, every 400 years, we've
missed an extra day.
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So now, every 400 years, we
reverse that rule, and we add
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the leap year back in.
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