Reasons for the seasons - Rebecca Kaplan

Transcriber: Andrea McDonough Reviewer: Jessica Ruby

翻译人员: Yunfan Wang 校对人员: Luyao Zou

When I was a kid,

我小时候，

my understanding of the seasons

对季节的理解

was that December and January were cold

是十二月和一月很冷

and covered with snow,

且银妆素裹，

April and May were bursting with flowers,

四月和五月繁花似锦，

July and August were hot and sunshiny,

七月和八月骄阳似火，

and September and October were a kaleidoscope of colorful leaves.

九月和十月则像充满缤纷秋叶的万花筒，

It was just the way the world worked,

世界如此运行，

and it was magical.

神奇美好。

If you had told me back then

如果你告诉那时的我，

that one-third of Earth's population

世界上有三分之一的人从未见过雪；

had never seen snow

or that July 4th was most definitely not a beach day,

或者，7月4号不是一个适合在海边晒太阳的日子，

I would have thought you were crazy.

我一定会觉得你疯了。

But in reality, seasonal change with four distinct seasons

但是事实上，地球上只有两个地区

only happens in two regions on the planet.

一年四季，四季分明。

And, even in those two,

而且，在这两个地区，

the seasons are reversed.

季节是相反的。

But why?

为什么呢？

A lot of people have heard of an astronomer

很多人听说过

called Johannes Kepler

天文学家约翰内斯·开普勒

and how he proved that planetary orbits are elliptical

以及他如何证明行星轨道是椭圆的，

and that the sun is not at the center of the orbit.

以及太阳不是轨道的中心。

It was a big deal when he figured this out

几百年他发现这些的时候，

several hundred years ago.

引起了巨大轰动。

His discovery solved a lot of mathematical problems

他的发现解决了

that astronomers were having

很多天文学家在计算行星轨道时遇到的问题。

with planetary orbit measurements.

While it's true that our orbit's not perfectly circular,

尽管行星轨道确实不是正圆，

those pictures in our science books,

那些自然课本、电视和电影里的，

on TV, and in the movies

关于地球轨道的图片，

give an exaggerated impression

夸大了地球轨道的扁长程度 。

of how elongated our orbit is.

In fact, Earth's orbit is very nearly a perfect circle.

实际上，地球的轨道几乎是正圆的，

However, because Earth's orbit is technically an ellipse,

但是，由于严格意义上地球的轨道是椭圆，

even though it doesn't look like one,

（尽管看起来并不像椭圆），

and the sun isn't quite exactly at the center,

而且太阳也并不是精准的位于轨道中心，

it means that our distance from the sun

这意味着我们与离太阳的距离，

does change through the year.

在一年之中不断变化。

Ah-ha!

啊－哈！

So, winter happens when the Earth is further away from the sun!

所以，当我们离太阳远一些的时候，冬天就来了！

Well, no, not so fast.

等等，并不是的这样，不要这样快下结论。

The Earth is actually closer to the sun

其实，地球与太阳的距离，

in January than we are in July

在一月比七月少五百万千米。

by 5 million kilometers.

January is smack-dab in the middle

对于那些北半球的人来说

of the coldest season of the year

一月刚好是最冷的月份。

for those of us up north.

Still not convinced?

还没有被说服吗？

How about this:

思考一下：

Summer and winter occur simultaneously

在地球上夏天和冬天可以同时存在。

on the surface of our planet.

When it's winter in Connecticut,

康涅狄格州（美国东北部）冬天时，

it's summer in New Zealand.

南半球的新西兰则是夏天。

So, if it's not the distance from the sun,

所以，季节跟地球和太阳的距离无关，

what else could it be?

那么，是什么因素导致了四季变化？

Well, we need to also need to know

我们还应该知道，

that the Earth doesn't sit straight up.

地球并不是"竖直"的，

It actually tilts.

地球是倾斜的。

And that axial tilt of the Earth

倾斜的地轴是造成四季的主要原因之一。

is one of the main reasons for the seasons.

The Earth spins on an axis

地球绕着地轴自转，

that's tilted 23.5 degrees from vertical.

地轴与垂直于轨道平面的直线 的夹角为23.5度。

At the same time, the Earth revolves around the sun

与此同时，地球绕太阳公转，

with the axis always pointing in the same direction in space.

且地轴永远指向宇宙的同一方向。

Together with the tilt,

地轴倾角、自转与公转的共同作用，

the spinning and revolving causes the number

导致了一年中同一地区的白昼时长

of hours of daylight in a region to change

随着时间推进而不断变化。

as the year goes by,

with more hours in summer

夏天白昼较长，

and fewer in winter.

冬天白昼较短。

So, when the sun is shining on the Earth, it warms up.

所以，当阳光照射在地球上，地球变暖。

After the sun sets, it has time to cool down.

日落后，地球开始降温。

So, in the summer,

所以，夏天时，

any location that's about 40 degrees north of the equator,

所有在北纬40度左右的地区，

like Hartford, Connecticut,

例如哈特福特市（美国康涅狄格州首府），

will get 15 hours of daylight each day

每天会有十五小时白昼，

and 9 hours of darkness.

以及九小时黑夜。

It warms up for longer than it cools.

加热时间长于冷却时间，

This happens day after day,

同种情形，日积月累，

so there is an overall warming effect.

导致整体效应为升温。

Remember this fact for later!

记住这个知识点，后面我们会再提到！

In the winter, the opposite happens.

在冬季，一切刚好相反。

There are many more hours of cooling time

冷却比加热时间更长。

than warming time,

and day after day, this results in a cooling effect.

所以日积月累，导致了冷却效应。

The interesting thing is, as you move north,

有趣的是，随着我们向北移动，

the number of daylight hours in summer increases.

夏天白昼时长增加。

So, Juneau, Alaska would get about 19 hours of daylight

所以，美国阿拉斯加州的首府朱诺市， 会有19小时白昼。

on the same summer day that Tallahassee, Florida gets about 14.

同一天，美国佛罗里达州首府塔拉哈西市， 只会有14小时日照。

In fact, in the summertime at the North Pole,

实际上，在北极，

the sun never sets.

夏天太阳从不落下。

OK, then, it's all about daylight hours, I've got it!

耶，我懂啦！ 所以是四季形成是由于白昼时长，

Well, no, there's another important piece to this puzzle.

等等，想要解出这道题， 还需要一个重要的知识点。

If daylight hours were the only thing

如果白昼时间是决定平均温度的唯一因素，

that determined average temperature,

wouldn't the North Pole be the hottest place

那么北极为什么不是夏天最热的地方？

on Earth in northern summer

because it receives 24 hours of daylight

要知道，在夏至左右的那些月份里，

in the months surrounding the summer solstice?

北极24小时白昼啊！

But it's the North Pole.

可是，它仍旧是北极，

There's still icebergs in the water

水中仍有冰川，

and snow on the ground.

地上仍有积雪。

So, what's going on?

所以，发生了什么呢？

The Earth is a sphere

因为地球是一个球体，

and so the amount of solar energy an area receives

所以地球上任一区域接收的太阳能，

changes based on how high the sun is in the sky,

随着太阳高度的变化而变化。

which, as you know, changes during the day

众所周知，从日出到日落，

between sunrise and sunset.

太阳的高度是不断变化的。

But, the maximum height also changes during the year,

此外，一年中太阳的最大高度也在不断变化。

with the greatest solar height during the summer months

夏天太阳高度高于其他季节，

and highest of all at noon on the summer solstice,

在夏至日的中午，太阳高度达到极大值。

which is June 21st in the northern hemisphere

北半球的夏至日是6月21日，

and December 21st in the southern hemisphere.

南半球的夏至日是12月21日。

This is because as the Earth revolves,

由于地球绕太阳公转，

the northern hemisphere ends up tilted away

倾斜的地轴导致了北半球在冬天相对背向太阳，

from the sun in the winter

and toward the sun in summer,

而在夏天相对朝向太阳，

which puts the sun more directly overhead

这使得夏天太阳位于正上方的时间变长。

for longer amounts of time.

Remember those increased summer time daylight hours?

还记得前面讲过夏天白昼渐长么？

And solar energy per square kilometer increases

随着太阳升高，

每平方公里接收的太阳能增加。

as the sun gets higher in the sky.

So, when the sun's at an angle,

所以传导到每一个方格上的能量，

the amount of energy delivered

to each square of the sunlit area is less.

太阳斜射时会少于太阳直射。

Therefore, even though the North Pole is getting 24 hours

因此，尽管北极在24小时白昼中不断升温，

of daylight to warm up,

the sunlight it receives is very spread out

但是北极接收的阳光非常分散，

and delivers less energy than a place further south,

而且传导的能量比南方的少，

where the sun is higher in the sky

因为南方地区太阳高度更大，

because it's more tilted toward the sun.

而且南方因地轴倾斜而朝向太阳。

Besides, the North Pole has a lot to make up for.

此外，北极的寒冷还因为，

It was cooling down without any sunlight at all

北极在六个月的冬季里， 没有阳光，逐渐降温。

for 6 months straight.

So, as the seasons change, wherever you are,

所以，不论你在哪里，当季节更替，

you can now appreciate not just the beauty of each new season

你不但可以欣赏每一个新的季节的美好，

but the astronomical complexity

而且还可以领悟带给你季节更替的

that brings them to you.

宇宙的奥妙 ！