每隔一段时间，就会有一个闰年。大多数人都认为闰年每四年发生一次，但其实这并不十分准确。计算哪一年会有闰年，哪一年没有闰年，可能比你想象得要复杂。本文探究为什么闰年不是每四年一次。

词汇：astronomy 天文学

2020 is a leap year, and as we all know leap years happen every four years, right? Wrong. That extra day in February is designed to keep the Earth in sync with the astrological or seasonal year. But, what you might not realise is that the maths isn’t that simple and occasionally we have to wait eight years.

The method behind calculating when leap years occur might be a bit more complicated than you think. Way back in 1582 the calendar we now use was first implemented. The Gregorian calendar was named after the pope at the time, Gregory X1V. The people who created it realised it wasn’t perfect to just add an extra day every four years. 

Here’s where the maths comes in. A day, of course, is measured by the amount of time it takes for the Earth to complete one full rotation on its axis. A year is the number of days it takes the Earth to orbit the Sun, which most people assume is 365. However, the Earth actually takes just under 365 and a quarter days to complete one full revolution around the Sun.

So, every four years we get an extra day, which falls on the 29th February. However, as mentioned earlier, it is in fact just under a quarter of a day. So, by rounding it up, we put the calendar out of sync again – which means that after a while we have too many days.

That imbalance has to be readdressed, otherwise after 400 years you’d end up with three extra days. So back in 1582, they realised that every turn of a century should skip their extra day to subtract those three extra days. This is why the years 1700, 1800, and 1900 weren’t leap years.

But wait, there’s more. The year 2000, the millennium, was a leap year. That’s because over a period of four hundred years we only need to remove three days. So, every 400 years the turn of the century is a leap year. This means the next time you hear someone saying ‘leap years happen every four years’ you can tell them why they’re wrong.

词汇表

in sync 同步的
astrological 占星学的
seasonal year 一年四季
calculate 计算
calendar 日历
measure 测量
rotation 旋转一圈
axis 轴线，中心线
orbit 轨道
revolution 旋转
round up 把数字调高成为整数
out of sync 不同步
imbalance 不均衡
turn of a century 世纪之交
millennium 千禧年

测验与练习

1.  阅读课文并回答问题。

1. True or False? Pope Gregory XII created the Gregorian calendar in 1582.

2.  How do we measure a day?

3. True or False? The Earth takes 365 and a quarter days to orbit the Sun.

4. If every four years was a leap year, how many days would the calendar be out of sync by after 400 years?

5. When is the next leap year that falls in the first year of a new century?

2. 选择意思恰当的单词或词组来完成下列句子。

1.The _______ to work out the number of days in a year is based on the amount of time it takes the Earth to go round the Sun.

calculate           calculating           calculation           calculate

2. We must _______ the changes to the schedule.

take                   takes                   took                     taken

3. It takes the Earth around 365 and a quarter days to _______ around the Sun.

revolve             revolution            revolutionise        revolted

4. We _______ up the bill so that the waiting staff got some extra money. 

rounding           rounds                round                   rounded

5. I'm not sure if it has been _______ properly. The numbers seem off.

measure           measured           measurement      measures

答案

1.  阅读课文并回答问题。  

1. True or False? Pope Gregory XII created the Gregorian calendar in 1582.
False. While the calendar was named after him, it wasn’t him who created it.

2. How do we measure a day?
A day is measured by the amount of time it takes for the Earth to complete one full rotation on its axis.

3. True or False? The Earth takes 365 and a quarter days to orbit the Sun.
False. The exact number is not a quarter. This is why extra calculations are needed.

4. If every four years was a leap year, how many days would the calendar be out of sync by after 400 years?
After 400 years, there would be three extra days if every four years was a leap year.

5. When is the next leap year that falls in the first year of a new century? 
2400. 2100, 2200, and 2300 will all skip leap years.

2 . 选择意思恰当的单词或词组来完成下列句子。

1. The calculation to work out the number of days in a year is based on the amount of time it takes the Earth to go round the Sun.

2. We must implement the changes to the schedule.

3. It takes the Earth around 365 and a quarter days to revolve around the Sun.

4. We rounded up the bill so that the waiting staff got some extra money.

5. I'm not sure if it has been measured properly. The numbers seem off.