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.

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

**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

**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?**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.*

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.

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