(单词翻译:单击)
To understand leap year you have to know this one fact: Earth rotates 365.24219 times during one full orbit of the sun.
要了解闰年，你得知道一个事实：那就是，围绕太阳周转一次，地球的自传次数为365.24219次。
That's right, a year is not 365 days. It's 365.24219 days.
没错，一年不是365天。而是365.24219天。
Which means our 365-day calendar is off by a quarter of a day, or about 6 hours, every year.
这也意味着我们标着一年365天的日历每年都少算了四分之一天，也就是6个小时左右。
It's pretty annoying.
怪烦人的。
Julius Caesar thought so too.
尤利乌斯•凯撒也是这么想的。
Back in 45 BC, he came up with a solution:
所以，早在公元前45年，他就想出了一个办法：
For three years, we ignore that extra quarter of a day,
在前三年的时间里，先忽略掉那额外的四分之一天，
then every four years, we add those four quarter days together and stick an extra day at the end of February.
到了第四年，再将这四个四分之一天加在一起，放在二月底。
Without that leap day, the gap between the calendar year and the true year would gradually increase.
如果没有闰年，日历上的一年和真实的一年之间的差距就会越来越大。
After 100 years, the calendar would be about 25 days off.
100年后，日历就会少25天左右。
So why does this matter?
这个问题重要在哪里呢？
Billions of years ago something smashed into earth and knocked its axis about 23 degrees.
数十亿年前，有东西撞到了地球，将地球的轴线撞歪了23度左右。
And thus were born the seasons.
四季就这样诞生了。
Winter when we're tilted away from the sun and summer when we're tilted toward it.
当我们转到背离太阳的一面时就是冬天，而转到面临太阳的一面就是夏天。
And we're used to these seasons lining up with our calendar.
而且我们已经习惯了这些季节与我们的日历相一致了。
It's handy for all sorts of record-keeping.
因为这样做各种记录都会很方便。
But without leap year, our calendar would become disconnected from the seasons.
然而，如果没有闰年的话，我们的日历就会与季节脱节。
After 700 years or so, Christmas would show up in the middle of summer.
而且约700年后，圣诞节就会出现在夏季。
We'd have Thanksgiving in the spring, St. Patrick's Day in the fall, and Labor Day in the winter. Chaos!
感恩节则会出现在春季，圣帕特里克节秋季，劳动节冬季。那岂不是乱成了一团！
It's still not a perfect system though.
然而，我们现在的日历系统也并非无可指摘。
Leap year actually overcorrects slightly, so every 100 years we skip a leap day,
事实上，我们的闰年略有修正过度的嫌疑，所以每100年，我们就跳过一个闰日，
but that puts the calender slightly behind again, so we skip skipping leap day every 400 years.
但这样一来日历就会稍稍落后，所以我们每400年就重新添加一个闰日。
And even still, after a few thousand years we'll be off by a day.
尽管如此，几千年后，我们的日历和回归年之间还是会差一天。
