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.�
I love when I see those, they really are great and concise way to learn new thing, or relearn some things to a bigger extent
Well I think the learning I just did justifies me skipping class today.
This guy always makes such great short educational videos.
Loved the Starcraft!
Nitpick alert: the length of the solar day varies by as much as 48 seconds throughout the year (it's longer closer to perihelion due to the earth having traveled farther in its orbit during a ~24 hour period, and having to therefore turn more to get back to facing towards the sun - axial tilt is also a factor). It's the AVERAGE solar day which varies by only a few milliseconds.
(I left this comment on the video too, but since OP is the maker of the video, I figured he'd be more likely to see it here.)
Well done!
As someone born on leap year I appreciate this video. I thought I knew everything about my birthday, but come to find out my birthday was almost skipped 12 years ago (2000). TIL.
I was expecting leap second to be a part of the video. But it wasn't. So educate yourselves even further and have a look at the wiki link.
MindOfMetalAndWheels, if you are actually CGP Grey on Youtube (and judging by your Reddit posts, you are), please know that I love your videos. They're concise, clear, and entertaining. Please keep up the good work. Edit: Grammar