my understanding of the seasons

我對季節的理解

was that December and January were cold

就是十二月和一月份很冷

and covered with snow,

白雪覆蓋大地；

April and May were bursting with flowers,

四五月百花盛開；

July and August were hot and sunshiny,

七八月炎熱、陽光普照；

and September and October were a kaleidoscope of colorful leaves.

九月十月，則像是各種顏色的樹葉組成的萬花筒。

It was just the way the world worked,

這就是世界運行的方式，

and it was magical.

多麼神奇。

If you had told me back then

如果你反過來告訴我

that one-third of Earth's population

地球上有三分之一的人口

had never seen snow

從沒看過雪、

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

或是七月四號對多數人來說 不適合去海灘玩，

I would have thought you were crazy.

我一定會覺得你瘋了。

But in reality, seasonal change with four distinct seasons

但事實上，所謂的四季分明

only happens in two regions on the planet.

只在這星球的兩個區域出現。

And, even in those two,

就算在這兩者中

the seasons are reversed.

季節也是顛倒的。

But why?

為什麼呢？

A lot of people have heard of an astronomer

許多人可能聽過一位天文學家，

called Johannes Kepler

他叫刻卜勒 (Johannes Kepler)，

and how he proved that planetary orbits are elliptical

他證明行星軌道是橢圓的

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

而且太陽並不在這軌道的中心。

It was a big deal when he figured this out

他這個想法

several hundred years ago.

在數百年前很驚人。

His discovery solved a lot of mathematical problems

這個發現解決許多數學上的問題，

that astronomers were having

那些天文學家們

with planetary orbit measurements.

在測量行星軌道時所面臨的問題。

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

由於地球的軌道不是個完美的圓形，

those pictures in our science books,

所以那些科學書籍裡的圖片，

on TV, and in the movies

或是電視上、電影片的，

give an exaggerated impression

都給我們誇大的印象，

of how elongated our orbit is.

告訴我們的軌道有多扁平。

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

事實上，地球的軌道跟正圓相去不遠。

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

然而，因為地球的軌道嚴格來說是個橢圓

even though it doesn't look like one,

──儘管它看起來不像──

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

同時太陽也不剛好就在中心，

it means that our distance from the sun

這表示我們到太陽的距離

does change through the year.

會在一整年中不斷變化。

Ah-ha!

啊哈！

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

所以冬天就是是地球離太陽最遠的時候！

Well, no, not so fast.

噢不，別太早下定論。

The Earth is actually closer to the sun

實際上地球在一月份的時候

in January than we are in July

比七月份的時候更靠近太陽，

by 5 million kilometers.

兩者相差 500 萬公里。

January is smack-dab in the middle

一月恰恰

of the coldest season of the year

恰恰就是一年之中最冷季節的中間，

for those of us up north.

對我們北半球來說是如此。

Still not convinced?

還是沒辦法說服你嗎？

How about this:

想想看這個：

Summer and winter occur simultaneously

夏季和冬季在這地表上

on the surface of our planet.

會同時發生。

When it's winter in Connecticut,

當康乃狄克是冬天時，

it's summer in New Zealand.

紐西蘭是夏天。

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

所以，如果跟太陽的距離無關，

what else could it be?

那還有什麼可能？

Well, we need to also need to know

嗯，我們還需要知道

that the Earth doesn't sit straight up.

地球並不是直立的。

It actually tilts.

它有點傾斜。

And that axial tilt of the Earth

而地軸傾斜

is one of the main reasons for the seasons.

是造成四季的主要原因之一。

The Earth spins on an axis

地球繞著地軸自轉，

that's tilted 23.5 degrees from vertical.

而地軸和鉛直線偏差 23.5 度。

At the same time, the Earth revolves around the sun

同時，地球也繞著太陽公轉，

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

而地軸總是指向宇宙中的同一個方向。

Together with the tilt,

搭配上地軸傾斜，

the spinning and revolving causes the number

自轉和公轉就造成了

of hours of daylight in a region to change

一年之中各地區

as the year goes by,

白日時數的變化。

with more hours in summer

夏天白晝長、

and fewer in winter.

冬天白晝短。

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

所以當陽光照耀著大地時，地球就開始升溫。

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

太陽下山後，地球就開始降溫。

So, in the summer,

因此，在夏天、

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

大約北緯 40 度的地區，

like Hartford, Connecticut,

像是康乃狄克州的哈特福，

will get 15 hours of daylight each day

每天會有 15 小時的白晝、

and 9 hours of darkness.

還有 9 小時的黑暗。

It warms up for longer than it cools.

升溫的時間比降溫的時間長。

This happens day after day,

日復一日，

so there is an overall warming effect.

就造成整體氣溫上升。

Remember this fact for later!

把這件事記住，等等會用到！

In the winter, the opposite happens.

在各天，則是相反的機制。

There are many more hours of cooling time

降溫的時數

than warming time,

比起升溫的時數多了好幾小時，

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

日復一日，就造成氣溫下降。

The interesting thing is, as you move north,

有趣的是，當你愈往北移，

the number of daylight hours in summer increases.

夏天的日照時數就愈長。

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

所以阿拉斯加州的朱諾有將近 19 小時的白畫、

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

而同一時間的夏日，佛羅里達州的塔拉赫西只有 14 小時的白晝。

In fact, in the summertime at the North Pole,

事實上，在北極的夏天

the sun never sets.

太陽是不會下山的。

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

好吧，就是日照時數的關係，我懂了！

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

嗯，不完全！這謎團裡還有另一個要素。

If daylight hours were the only thing

如果日照時間

that determined average temperature,

是影響平均氣溫的唯一因素，

wouldn't the North Pole be the hottest place

那在北半球的夏天，

on Earth in northern summer

北極不就應該是最熱的地方嗎？

because it receives 24 hours of daylight

因為在夏至附近的月份，

in the months surrounding the summer solstice?

北極都接收了 24 小時的日照。

But it's the North Pole.

但那裡是北極。

There's still icebergs in the water

海上還是有冰山、

and snow on the ground.

陸上還是有雪。

So, what's going on?

是怎麼回事呢？

The Earth is a sphere

地球是個球體，

and so the amount of solar energy an area receives

所以一塊區域所接收到的太陽能量

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

會隨著太陽在天空中的高度而變化，

which, as you know, changes during the day

如你所知，

between sunrise and sunset.

這高度會在日出日落間變化。

But, the maximum height also changes during the year,

但是，最大的高度同時也會在一年之中有變化，

with the greatest solar height during the summer months

夏天的時候太陽較高

and highest of all at noon on the summer solstice,

而在夏至正午的時候達到最高，

which is June 21st in the northern hemisphere

也就是北半球的七月二十一號、

and December 21st in the southern hemisphere.

南半球的十二月二十一號。

This is because as the Earth revolves,

這是因為隨著地球的公轉

the northern hemisphere ends up tilted away

北半球在冬天傾斜的方向

from the sun in the winter

會較遠離太陽，

and toward the sun in summer,

而在夏天會較靠近太陽，

which puts the sun more directly overhead

這讓太陽有更長的時間、

for longer amounts of time.

也更能容易直射下來。

Remember those increased summer time daylight hours?

還記得夏天那較多的日照時數嗎？

And solar energy per square kilometer increases

當太陽愈高掛天空的時候，地球上每平方公里

as the sun gets higher in the sky.

所接收到的太陽能量就愈多。

So, when the sun's at an angle,

所以當太陽沒直射時，

the amount of energy delivered

每塊區域接收到的能量

to each square of the sunlit area is less.

就愈少。

Therefore, even though the North Pole is getting 24 hours

因此，儘管北極有 24 小時的白晝

of daylight to warm up,

可以加熱升溫，

the sunlight it receives is very spread out

但它接收到的陽光很分散，

and delivers less energy than a place further south,

同時傳遞的能量也比南方的少，

where the sun is higher in the sky

這是因為南方傾斜的方向較靠近太陽，

because it's more tilted toward the sun.

所以太陽在天空中較高。

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

此外，北極寒冷還有其它原因。

It was cooling down without any sunlight at all

它有 6 個月都不會接收到陽光，

for 6 months straight.

因此降溫。

So, as the seasons change, wherever you are,

所以當季節變化的時候，不論你身在何處，

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

你現在不只可以欣賞每個新季節的美麗

but the astronomical complexity

同時還有帶給你這些變化的

that brings them to you.

宇宙的奧妙。