At first we just needed to have an idea of what season it was, and maybe what season was coming up next, and around when.

起初我們只需要知道季節的轉換，或許知道下個到來的季節是什麼，和大概是什麼時間來。

But after the invention of agriculture, it started being important to keep track of each day of the year.

但在農業出現之後，記載每一天開始變得很重要。

Once you start trying to count the days of the year though, it doesn't take long to notice a problem: you just can't make a consistent calendar with the same number of days every year, because there aren't a whole number of days in the year.

當你開始數一年有幾天時，不用多久就會發現一個問題：你每年的日曆無法一致，每年的天數都不一樣，因為一年本來就不是由完整的天數構成。

That is why today's date exists!

這就是為什麼 2 月 29 日存在！

We have to add an extra day to the calendar every now and then and we do it on February 29th.

我們需要偶爾在日曆上加上額外的一天，而這就是 2 月 29 日的由來。

The main issue is that the Earth's orbit and its rotation don't have anything to do with each other.

主要的問題在於地球公轉與自轉兩者之間沒有關係。

When our planet spins once around on its axis we call that a day, and when it completes a full orbit of the sun, we call that a solar year.

當我們的星球以地軸為中心自轉一圈為一日，完整繞太陽公轉一圈為一個太陽年。

Now it'd be nice if Earth always got back to the same place in its orbit after a set number of complete rotations, which would be an exact number of days, but it doesn't.

如果地球在可以一定次數的自轉後回到公轉軌道上的同一位置，這樣一年就會是一定的天數，但事實並非如此。

Earth orbits the sun once every 365.2425 rotations, so if your calendar has a whole number of days, like 365 or 366, then you're counting either too few days or too many.

地球公轉太陽一圈會自轉 365.2425 圈，所以如果將一年定為 365 或 366 天的話不是會太多就是太少。

And if you're counting wrong, then things like the solstices, the longest and shortest days of the year, which always happen at the same point in the Earth's orbit, will move around the calendar.

所以如果你數錯了，那一年之中當地球公轉至軌道上某一特定位置，也就是最長跟最短的夏至與冬至發生時，將不會固定於日曆上的某一天。

That makes it a little tricky to plan for events that you want to happen on the same day every year, say a solstice celebration.

這會讓一些希望在每年同一時間舉辦的活動在安排上有些麻煩，好比說夏至或冬至慶典。

And if that wasn't complicated enough, a lot of ancient cultures, like the Babylonians, Chinese, and Romans, used calendars based on the moon.

如果說這些還不夠複雜的話，其他古老的文明如巴比倫、中國與羅馬，則使用依據月亮排程的曆法。

But, the behavior of the moon, again, has nothing to do with our rotation or our orbit.

但一樣，月亮的移動與我們的自轉與公轉無關。

The moon goes through a little more than 12 cycles, each a little more than 29 days, in the time that it takes for the Earth to go around the sun.

月亮在地球公轉太陽一圈的同時，公轉地球 12 多圈，公轉一圈又花 29 天又多一點點。

And that means they had a similar problem: 12 lunar months a year is too few, but 13 lunar months in a year is too many.

而這表示了他們有著相同的問題：12 個陰曆月對於一年來說太少，但 13 個陰曆月又太多。

So most people using a lunar calendar would have 12 months most years but then, they'd add an extra month every couple of years to keep things, like harvest, around roughly the same solar date.

所以大部分使用陰曆的人在大部分時間會是一年 12 個月，但他們會每幾年多加一個月，將活動固定在與陽曆差不多的時間的時間，例如收割季。

Then there were the Romans, always trying to keep things interesting.

但這時羅馬人呢，他們總是喜歡讓事情變得有趣。

Instead of having a system in place, for how often there should be an extra month, they left it up to their highest religious authority: the Pontifex Maximus.

與其研究究竟該在多少時間後應該多加一個月以便將整個曆法系統固定下來，他們將整件事交由羅馬宗教權限最高的「大祭司」來決定。

Which kinda worked for a while, at least until Julius Caesar came along.

整件事至少直到凱薩大帝的出現之前都算是有在運作。

The Romans thought adding those extra months was super bad luck, so they didn't want to do it while they were at war, and Julius was at war all the time.

羅馬人認為加那些額外月數是很衰的事情，所以他們不想在打仗的時候加，但凱薩一天到晚都在打仗。

So, the calendar got way off from where it should have been.

所以，日曆大大偏移了原本應該要呈現的樣子。

So eventually, Caesar decided that the Romans should just use a brand new calendar, that relied on a set system of when and where to add extra days.

最終，凱薩決定羅馬人需要有一個全新的曆法，依據一套決定每隔多久要加額外的月數，與加在何處的固定系統。

And that's how we got the Julian Calendar, which Western Europe and Christian churches used for a really long time.

而這就是西歐與基督教會使用了很久的儒略曆。

Most Julian years had 365 days, but like we said earlier, the year is about a quarter of a day longer than that, so every 4th year got an extra 366th day.

儒略曆大部分時間是一年 365 天，但就如我們早些時間所說，一年是 365 又四分之一天，所以每四年會有額外的一天。

Seems like a good fix, but things are not that simple.

看起來是個好的解決辦法，但事情沒有這麼簡單。

The four year system ends up slightly over-estimating the length of the year by about 10 minutes, something the designers of the Julian calendar knew about, but they decided it wasn't worth complicating the calendar over.

這個四年的系統導致一年的時間被多預估了 10 分鐘，設計儒略曆的人知道這件事情，但他們決定這多估的 10 分鐘不值得把整個日曆變得更複雜。

It's 10 minutes a year, what could go wrong?

就一年中的 10 分鐘，還可以出多大的差錯呢？

Well, 10 minutes a year, over a lot of years, can really add up.

一年就 10 分鐘，過了很多年這些多出的時間就變得很可觀。

By the late 1500s, around 1600 years after Caesar, the calendar was about 13 days off from where it should have been.

在 1500 年代末期，凱薩大帝年代後約 1600 年，這個曆法偏移了約 13 天。

That's when Pope Gregory XIII, the Pontifex Maximus of the Catholic Church at the time, decided that the Julian calendar needed an overhaul.

這時天主教會的大祭司，教宗格列高利十三世，決定要徹底的大修改儒略曆。

So he put together the Gregorian calendar, the one that we still use today, which is really just the Julian Calendar with a couple of changes.

他拼湊出了我們現在還在使用的格里曆 (公曆)，其實就是做了點小修改的儒略曆。

The Gregorian calendar's new rules were every four years add an extra day, except, don't add it when the year is divisible by 100 except, actually, do add it if the year is divisible by 400.

公曆的新規則為每四年加額外的一天，年數無法被 100 整除時不要加上額外的天數，但在能被 400 整除時加上。

So 1700, 1800, and 1900, weren't leap years because they were all divisible by 100, but not by 400.

所以 1700、1800、1900 不是閏年因為他們可以被 100 整除，但無法被 400 整除。

The year 2000 was a leap year since 2000 is divisible by 400.

2000 年是閏年因為可以被 400 整除。

And, of course, 2016 is a leap year as well.

當然，2016 年也是閏年。

If we were still using the Julian calendar, 2016 would still be a leap year.

如果我們還在使用儒略曆，那 2016 年就不會是閏年。

But the calendar would now be behind by 16 days.

但日期會比原本應該的曆法早 16 天。

So this video would have been uploaded on February 13th, 2016 not February 29th, so it wouldn't have made any sense.

而這支影片就會是在 2016 年 2 月 13 號，而不是 2 月 29 日上傳，這一切都不合理。

But the universe is complicated and does not care about our calendars.

但宇宙一直都是很複雜的，也不在乎我們的曆法。

So the Gregorian calendar is still a little bit too long.

所以其實公曆預估的時間其實還是有些太長。

We will add a day about every 7,700 years, but don't hold your breath for a new calendar.

我們會每 7700 年多加一天，但可不要眼巴巴地等著新的曆法出現。

The Gregorian calendar was only introduced when the Julian one was 13 days off from the true date.

公曆在儒略曆與真實日期偏差了 13 天之後才出現。

And that won't happen to the Gregorian calendar for another 100,000 years or so.

而這樣的情況近 100000 年在公曆都不會發生。

Thank you for watching this episode of SciShow, which was brought to you by our patrons on Patreon.

謝謝收看本集的 SciShow, 由我們來自 Patreon 的合作夥伴呈現。

If you want to help support the show, you can go to: patreon.com/SciShow

如果想要支持這個節目，你可以前往網址 patreon.com/SciShow。

We always very much appreciate that, it is the reason why we can do all this.

我們一直都非常感謝，你們的幫助是我們可以完成這一切的原因。

And if you want to keep getting smarter with us, as always, go to YouTube.com/SciShow and subscribe.

如果想要與我們一起變得更聰明，老樣子，訂閱 YouTube.com/SciShow。