Today we celebrate the Leap Year, the extra day at the end of February that comes around once every four years.
For some people the leap year has a special significance. This year, for instance, there have been initiatives to encourage people to put that extra day to use helping the planet.
But where does the idea of a leap year come from, and why exactly do we have one?
The second question is a bit easier to answer than the first. Many of you might recall from your school days that a year is not exactly 365 days. It actually takes the planet 365.242 days to complete a revolution around the sun. After four years, those extra .242 days add up to a whole extra day.
Even then it’s not exactly perfect. Obviously, .242 times four is not 1. We get around this problem by skipping a leap year for three out of four century years. So the year 2000 was a leap year, but the years 2100, 2200 and 2300 will not be.
Like many advancements in civilization, the concept of a leap year came from the Egyptians. The Egyptians first began using a calendar with a leap year during the Ptolemaic dynasty (300-30 B.C.E.) Many people credit Julius Caesar for the use of the leap year in his Julian calendar, but he almost certainly took the ideas from the Egyptians. One legend says that he took the idea from his lover Cleopatra.
When Caesar returned to Rome and implemented the calendar the leap year became popularized. It would remain unchanged until the 16th century A.D.
In 1582 Pope Gregory XIII introduced a new, but very similar, calendar. As we mentioned earlier, you actually add slightly too much time to the year by adding a day every four years. This results in an unnecessary extra day being added to the calendar once every 128 years. This annoyed Gregory. He believed that this extra day ever century and a quarter had resulted in Christian holidays being celebrated on the wrong days.
So Gregory went ahead and did something about it by inventing the Gregorian calendar, which we still use today. The Gregorian calendar introduced the concept of skipping three of four century years as a leap year, and so keeps the balance a bit better than the Julian calendar.
So there you are. Now that you know the science and history behind your extra day, what are you going to do on it?
译文:
闰年知多少
今天是闰年特有的日子。在闰年，二月份天数比普通年份多一天，每四年出现一次。
闰年对一些人而言有着特殊的意义，拿今年来说，就有人倡导大家用这多出来的一天为地球做点善事。
然而，闰年的概念源自哪里，而我们又究竟为何有这闰年一说呢？
后一个问题要比前一个好回答点。许多人可能会想起以前在学校里学到过:一年并不正好是365天，而事实上地球绕太阳一周需要365.242天，每年这多出来的0.242天四年下来就会累积成一天。
可是这样的解释还不到位，因为0.242乘以4显然不等于1。我们只有在400年中跳过三个闰年，这个问题才能解决。这样看来2000年是闰年，2100，2200，还有2300年都不是闰年。
像许多其它伴随文明进步得以发展的事物一样，闰年的概念源自埃及人。早在托勒密王朝（公元前300—30年）埃及人就在日历法中引用闰年这一说。许多人还因尤里乌斯·凯撒在他的儒略（西元前46年由凯撒创立的历法）中使用了闰年的概念而称颂他，但其实凯撒是完全借鉴了埃及人的历法思想。传说，凯撒大帝是从他情人克丽奥佩脱拉（埃及艳后）那里知道闰年的。
凯撒回到罗马后就开始实行儒历，闰年的概念也得以普及，这一历法一直沿用至公元16世纪。1582年罗马教皇格列高利十三世引进了一个新的却非常相似的历法。正如见面所提到的，事实上人们为每年多加一点时间使得四年多出一天来，这就使得每128年就会人为地多出来一天。这就惹恼了教皇格列高利，他觉得这每隔一个多世纪就会多出来的一天会使得基督教节日在错误的日期被庆祝。
因此教皇格列高利就创造了格列高利历来改进原先的计历方法。按照这个历法，一个世纪年如正好被400除尽即为闰年。与凯撒的儒略相比，格列高利历能更好地消除人为计历误差。
好了，现在你可知道这多出来的一天背后的科学原因和历史渊源了,那么你会在这特殊的一天里做些什么呢?
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