When I was a kid, my understanding of the seasons was that December and January were cold and covered with snow,
我小时候对季节的理解，是十二月和一月很冷且银妆素裹，
April and May were bursting with flowers, July and August were hot and sunshiny,
四月和五月繁花似锦，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, and it was magical.
这就是世界运行的方式，多么神奇。
If you had told me back then that one-third of Earth's population had never seen snow
如果你那时告诉我，世界上有三分之一的人从未见过雪；
or that July 4th was most definitely not a beach day, I would have thought you were crazy.
或者，7月4号不是一个适合在海边晒太阳的日子，我一定会觉得你疯了。
But in reality, seasonal change with four distinct seasons only happens in two regions on the planet.
但是事实上，地球上只有两个地区一年四季，四季分明。
And, even in those two, the seasons are reversed. But why?
而且，在这两个地区，季节是相反的。为什么呢？
A lot of people have heard of an astronomer called Johannes Kepler
很多人听说过天文学家约翰内斯·开普勒
and how he proved that planetary orbits are elliptical and that the sun is not at the center of the orbit.
以及他如何证明行星轨道是椭圆的，以及太阳不是轨道的中心。
It was a big deal when he figured this out several hundred years ago.
几百年他发现这些的时候，引起了巨大轰动。
His discovery solved a lot of mathematical problems that astronomers were having with planetary orbit measurements.
他的发现解决了很多天文学家在计算行星轨道时遇到的问题。
While it's true that our orbit's not perfectly circular,
地球的轨道确实不是个完美的圆形，
those pictures in our science books, on TV, and in the movies give an exaggerated impression of how elongated our orbit is.
所以那些科学书籍里的图片，或是电视上、电影中的，都给我们夸大的印象，告诉我们的轨道有多扁平。
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, even though it doesn't look like one,
但是，由于严格意义上地球的轨道是椭圆--尽管看起来并不像，
and the sun isn't quite exactly at the center, it means that our distance from the sun does change through the year.
而且太阳也并不是精准的位于轨道中心，这意味着我们与离太阳的距离，在一年之中不断变化。
Ah-ha! So, winter happens when the Earth is further away from the sun!
啊哈！所以，当我们离太阳远一些的时候，冬天就来了！
Well, no, not so fast.
噢不，别太早下定论。
The Earth is actually closer to the sun in January than we are in July by 5 million kilometers.
其实，地球与太阳的距离，在一月要比在七月少五百万公里。
January is smack-dab in the middle of the coldest season of the year for those of us up north.
对于那些北半球的人来说，一月刚好是最冷的月份。
Still not convinced? How about this: Summer and winter occur simultaneously on the surface of our planet.
还没有被说服吗？思考一下：在地球上夏天和冬天可以同时存在。
When it's winter in Connecticut, it's summer in New Zealand.
康涅狄格州冬天时，南半球的新西兰则是夏天。
So, if it's not the distance from the sun, what else could it be?
所以，如果跟太阳的距离无关，那还有什么可能？
Well, we need to also need to know that the Earth doesn't sit straight up. It actually tilts.
我们还应该知道，地球并不是“竖直”的，地球是倾斜的。
And that axial tilt of the Earth is one of the main reasons for the seasons.
倾斜的地轴是造成四季的主要原因之一。
The Earth spins on an axis that's tilted 23.5 degrees from vertical.
地球绕着地轴自转，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 with the axis always pointing in the same direction in space.
与此同时，地球绕太阳公转，且地轴永远指向宇宙的同一方向。
Together with the tilt, the spinning and revolving causes the number of hours of daylight in a region to change as the year goes by,
地轴倾角、自转与公转的共同作用，导致了一年中同一地区的白昼时长随着时间推进而不断变化，
with more hours in summer and fewer in winter.
夏天白昼较长，冬天白昼较短。
So, when the sun is shining on the Earth, it warms up. After the sun sets, it has time to cool down.
所以，当阳光照射在地球上，地球变暖。日落后，地球开始降温。

So, in the summer, any location that's about 40 degrees north of the equator, like Hartford, Connecticut,
所以，夏天时，所有在北纬40度左右的地区，例如哈特福特市，
will get 15 hours of daylight each day and 9 hours of darkness.
每天会有十五小时的白昼以及九小时的黑夜。
It warms up for longer than it cools.
升温的时间比降温的时间长。
This happens day after day, so there is an overall warming effect.
日复一日，就造成整体气温上升。
Remember this fact for later! In the winter, the opposite happens.
记住这个知识点，后面我们会再提到！在冬季，一切刚好相反。
There are many more hours of cooling time than warming time, and day after day, this results in a cooling effect.
降温的小时数比起升温的小时数要多好几个小时，日复一日，就造成气温下降。
The interesting thing is, as you move north, the number of daylight hours in summer increases.
有趣的是，随着我们向北移动，夏天白昼的时长会增加。
So, Juneau, Alaska would get about 19 hours of daylight on the same summer day that Tallahassee, Florida gets about 14.
所以，美国阿拉斯加州的首府朱诺市会有19小时的白昼。同一天，美国佛罗里达州首府塔拉哈西市只会有14小时。
In fact, in the summertime at the North Pole, the sun never sets.
实际上，在北极，夏天太阳从不落下。
OK, then, it's all about daylight hours, I've got it!
好吧，就是日照小时数的关系，我懂了！
Well, no, there's another important piece to this puzzle.
嗯，不，这谜团里还有另一个要素。
If daylight hours were the only thing that determined average temperature,
如果白昼时间是决定平均温度的唯一因素，
wouldn't the North Pole be the hottest place on Earth in northern summer
那么北极为什么不是夏天最热的地方，
because it receives 24 hours of daylight in the months surrounding the summer solstice?
要知道，在夏至附近的那些月份里，北极24小时白昼啊！
But it's the North Pole. There's still icebergs in the water and snow on the ground. So, what's going on?
可是，它仍旧是北极，水中仍有冰川，地上仍有积雪。所以，发生了什么呢？
The Earth is a sphere and so the amount of solar energy an area receives changes based on how high the sun is in the sky,
因为地球是一个球体，所以地球上任一区域接收的太阳能，随着太阳高度的变化而变化，
which, as you know, changes during the day between sunrise and sunset.
众所周知，从日出到日落，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 and highest of all at noon on the summer solstice,
夏天太阳高度高于其他季节，在夏至日的中午，太阳高度达到极大值。
which is June 21st in the northern hemisphere and December 21st in the southern hemisphere.
北半球的夏至日是6月21日，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 from the sun in the winter and toward the sun in summer,
倾斜的地轴导致了北半球在冬天相对背向太阳，而在夏天相对朝向太阳，
which puts the sun more directly overhead for longer amounts of time.
这使得夏天太阳位于正上方的时间变长。
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 as the sun gets higher in the sky.
随着太阳升高，每平方公里接收的太阳能增加。
So, when the sun's at an angle, the amount of energy delivered to each square of the sunlit area is less.
所以当太阳没直射时，每块区域接收到的能量就会少很多。
Therefore, even though the North Pole is getting 24 hours of daylight to warm up,
因此，尽管北极在24小时白昼中不断升温，
the sunlight it receives is very spread out and delivers less energy than a place further south,
但是北极接收的阳光非常分散，而且传导的能量也比南方的少，
where the sun is higher in the sky because it's more tilted toward the sun.
这是因为南方因地轴倾斜而朝向太阳，所以南方地区太阳高度更高。
Besides, the North Pole has a lot to make up for.
此外，北极寒冷还有其它原因。
It was cooling down without any sunlight at all for 6 months straight.
它有6个月都不会接收到阳光，逐渐降温。
So, as the seasons change, wherever you are,
所以当季节变化的时候，不论你身在何处，
you can now appreciate not just the beauty of each new season but the astronomical complexity that brings them to you.
你不但可以欣赏每一个新的季节的美好，同时还有带给你这些变化的宇宙的奥妙。