爲什麼會有四年一次的閏日？大綱
2020年是一個閏年，2月29日這個閏日即將登場。爲什麼會有閏年和閏日呢？其實，這是一個歷史遺留問題。而且，閏日也不是“四年等一回”那麼簡單。
At some point in elementary school, your science teacher probably explained to you that there are 365 days in a year because that’s how long it takes for Earth to complete one full rotation around the sun. What they might not have specified, however, is that it’s not exactly 365 days—it’s actually closer to 365.2421 days.
上小學時，你們的科學老師很可能會向你們解釋一年有365天，因爲這是地球繞太陽公轉一圈的時間。不過，他們可能沒有詳細說明，地球公轉一圈並不是精確的365天，實際上說是365.2421天更準確一些。
So, if we want our calendar year to begin right when Earth begins a new rotation around the sun, we have to account for (roughly) an extra quarter of a day each year, or one day every four years. reports that the Egyptians had already been doing this for a while before Europe finally caught on in 46 BC, when Roman dictator Julius Caesar and astronomer Sosigenes put their heads together to come up with what we now call the Julian calendar, which includes 12 months, 365 days, and an additional “leap day” every four years on February 29.
因此，如果我們希望年曆的開端正好是地球開始新一輪公轉的時候，我們就必須每年多算（大約）四分之一天，或每四年多算一天。歷史網報道稱，埃及人先採用了這種算法，後來到了公元前46年歐洲人才開始這麼算。當時羅馬獨裁者尤利烏斯·愷撒（愷撒大帝）和天文學家索西琴尼商討後製定了我們今天所謂的儒略曆，儒略曆共有12個月、365天，每四年加上一個閏日（2月29日）。
But rounding 0.2421 up to 0.25 each year created an issue, because it didn’t quite add up to a full day every four years—and that tiny discrepancy meant that after 128 years, the calendar year ended up starting a day before Earth had completed its rotation around the sun. By the 14th century, the calendar year was starting a whopping 10 days before Earth finished its orbit.
但是把每年的0.2421天約等於0.25天也產生了一個問題，因爲按照0.2421算的話，每四年要增加的並不是一整天。這個微小的差額意味着128年後，年曆就會在地球完成公轉前一天開始。到了14世紀，年曆開始的時間比地球完成公轉的時間早了10天之多。
In 1582, Pope Gregory XIII sought to correct the error by suggesting that we simply skip a leap day every so often. His Gregorian calendar, which we still use today, mandates that we omit the leap day during years evenly divisible by 100 but not by 400. For instance, the year 2000 included a leap day because it’s divisible by 100 and 400; the year 2100, on the other hand, will not include a leap day, since it’s evenly divisible by 100, but not by 400.
1582年，教皇格列高利十三世想要改正這一錯誤，於是建議我們時不時跳過一個閏日。他的格里曆（我們今天仍在使用的公曆）規定，在能夠被100整除但不能被400整除的年份跳過閏日。舉例來說，2000年有一個閏日，因爲2000能被100和400整除；而2100年沒有閏日，因爲它能被100整除，但不能被400整除。
Gregory XIII’s correction to Caesar’s overcorrection is itself a bit of an under-correction, so we’ll probably need to reevaluate our leap day protocol again in about 10,000 years.
教皇格列高利十三世對愷撒大帝過度校正的更正本身是一種校正不足，所以大約1萬年後，我們將很可能需要對我們的閏日規則進行重新評估。
