## Subtitles section Play video

• A calendar year is made of three hundred and sixty five days -- a number that refuses to

• 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.�

A calendar year is made of three hundred and sixty five days -- a number that refuses to

Subtitles and vocabulary

Click the word to look it up Click the word to find further inforamtion about it

# What is a Leap Year?

• 126 3
fisher posted on 2013/03/24
Video vocabulary