be divide nicely, which is why we end up with uneven months of either 30 or 31 days. Except

for February -- the runt of the litter -- which only gets 28... except when it gets 29 and

then the year is 366 days long.

Why does that happen? What kind of crazy universe do we live in where some years are longer

than others?

To answer this we need to know: just what is a year?

Way oversimplifying it: a year is the time it takes Earth to make one trip around the

sun. This happens to line up with the cycle of the seasons.

Now, drawing a little diagram like this showing the Earth jauntily going around the sun is

easy to do, but accurately tracking a year is tricky when you're on Earth because the

universe doesn't provide an overhead map.

On Earth you only get to see the seasons change and the obvious way to keep track of their

comings and goings is to count the days passing which gives you a 365 day calendar.

But as soon as you start to use that calendar, it slowly gets out of sync with the seasons.

And with each passing year the gap gets bigger and bigger and bigger.

In three decades the calendar will be off by a week and in a few hundred years the seasons

would be flipped -- meaning Christmas celebrations taking place in summer -- which would be crazy.

Why does this happen? Did we count the days wrong? Well the calendar predicts that the

time it takes for the Earth to go around the sun is 8,760 hours. But, if you actually timed

it with a stopwatch you'd see that a year is really longer than the calendar predicts

by almost six hours. So our calendar is moving ever-so-slightly faster than the seasons actually

change.

And thus we come to the fundamental problem of all calendars: the day/night cycle, while

easy to count, has nothing to do with the yearly cycle.

Day and night are caused by Earth rotating about its axis. When you're on the side faceing

the sun, it's daytime and when you're on the other side it's night. But this rotation is

no more connected to the orbital motion around the sun than a ballerina spinning on the back

of a truck is connected to the truck's crusing speed.

Counting the number of ballerina turns to predict how long the truck takes to dive in

a circle might give you a rough idea, but it's crazy to expect it to be precise.

Counting the days to track the orbit is pretty much the same thing and so it shouldn't be

a surprise when the Earth dosen't happen to make exactly 365 complete spins in a year.

Irritatingly, while 365 days are too few 366 days are too many and still cause the seasons

to drift out of sync, just in the opposite way.

The solution to all this is the leap year: where February gets an extra day, but only

every four years.

This works pretty well, as each year the calendar is about a quarter day short, so after four

years you add an extra day to get back in alignment.

Huzzah! The problem has been solved.

Except, it hasn't.

Lengthening the calendar by one day every four years is slightly too much, and the calendar

still falls behind the seasons at the rate of one day per hundred years.

Which is fine for the apathetic, but not for calendar designers who want everything to

line up perfectly.

To fix the irregularity, every century the leap year is skipped.

So 1896 and 1904 were leap years but 1900 wasn't.

This is better, but still leaves the calendar ever-so-slightly too fast with an error of

1 day in 400 years.

So an additional clause is added to the skip the centuries rule that if the century is

divisible by 400, then it will be a leap year.

So 1900 and 2100 aren't leap years, but 2000 is.

With these three rules, the error is now just one day off in almost eight thousand years

which the current calendar declares 'mission accomplished' and so calls it a day.

Which is probably quite reasonable because eight thousand years ago humans were just

figuring out that farming might be a good idea and eight thousand years from now we'll

be hopefully be using a calendar with a better date tracking system.

But perhaps you're a mathematician and a 0.0001 percent error is an abomination in your eyes

and must be removed.

"Tough luck" says The Universe because the length of a day isn't even constant. It randomly

varies by a few milliseconds and on average and very slowly decreases by about 1 millisecond

per hundred years. Which means it's literally impossible to build a perfect calendar that

lasts forever.

In theory the length of a day will expand to be as long as a curent month -- but don't

worry in practice it will take tens of billions of years, and our own expanding sun will destroy

the earth long before that happens.

Sorry, not quite sure how we got from counting the days of the months to the fiery unavoidable

end of all human civilization -- unless of course we have an adequately funded space

program (hint, hint) -- but there you have it.

For the next eight millennia Leap years will keep the calendar in sync with the seasons

but in a surprisingly complicated way.

You can learn a lot more about orbits, different kinds of years and supermassive black holes

and over at Minute Physics. As always, Henry does a great job of explaining it all in his

new video. Check it out.�