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为什么十二月是最长的

Why December Has The Longest Days

在北半球,十二月有最短的白天
In the Northern Hemisphere, December has the fewest
和最长的黑夜
hours of daylight and the most darkness because at that
因为那时地轴的倾斜
time the tilt of the Earth’s axis is pointing the Northern
使北半球远离太阳
Hemisphere away from the Sun. However, as
虽然听起来有点反直觉
counter-intuitive as it might sound, December actually
十二月其实拥有全年最长的日子。
has the longest days of the year. Modern clocks,
现代钟表,当然,规定每天有86480秒长,
of course, think every day is 86,400 seconds long,
但那只是就全年来说一个太阳日的平均长度
but that just the average length of a solar day over the
一个太阳日是你用日晷测量出的一天
course of the year. A solar day is what you actually
或者换种说法
measure with a sundial, or equivalently it’s the time it
它是地球的一条经线再次转为面向太阳
takes for a line of longitude on the Earth to rotate back
所需的时间
to face the Sun again. This actually requires slightly
这实际上需要稍微超过360度的旋转
more than 360 degrees of rotation because the Earth
因为地球不只是在空间自转,
isn’t just sitting in space rotating, it’s also moving
也绕太阳旋转所以它必须旋转约361度
around the Sun so it has to rotate roughly 361 degrees
在太阳回到恰好特定的位置之前
before the Sun comes back perfectly overhead a
在一段连续的时间内
particular place on consecutive days. If the Earth’s orbit
如果地球的轨道是完美的圆形且其地轴完全垂直
were perfectly circular and its axis were perfectly
那这个故事可以结束了
upright, that would be the end of the story. However, the
然而,地球的轨道是椭圆形,
Earth’s orbit is elliptical, so sometimes the Earth is
所以有时地球微微距太阳更近,
slightly closer to the Sun, and the peculiarities of gravity
而重力的独特性意味着它在离太阳越近的时候移动的越快
mean it moves faster when it’s closer to the Sun, so it
所以它在24小时内走的更快
goes farther around the Sun in 24 hours so the Earth has
所以地球需要在太阳回来前转得稍微更远些
to rotate slightly farther before the Sun comes back
精确地说,0.033度
right overhead. 0.033 degrees farther, to be precise.
更远的旋转需要更多的时间,所以当地球
More rotation takes more time, so when the Earth is
最接近太阳时,真正的日冕测量的一天
closest to the Sun, the real sundial measured day length
长度增长了约八秒。
is lengthened by about eight seconds. Plus the Earth’s
加上地球的轴线是倾斜的,这就有了四季
axis is tilted, which is what gives rise to the seasons,
但这也意味着
but also means that at the time of year when the tilt
当一年中倾斜点朝向或反向太阳时,小部分经线
points towards or away from the sun, narrower slices of
将直面太阳。所以
longitude are aimed directly at the sun. So as the Earth
当地球在轨道上运行时,它需要转的更远以便
moves in its orbit, it has to rotate slightly farther in order
特定的一部分经线
for a particular line of longitude to catch up with the
赶上太阳旋转的方向。
changing direction to the Sun. 0.088 degrees farther, to
精确的说,0.088度
be precise. And again more rotation takes more time, so
并且更远的旋转需要更多的时间,所以当地球斜对太阳时,实际的日晷
when the Earth is tilted towards the Sun, the real sundial
测量的一天长度延长了大约21秒。
measured day length is lengthened by about 21
现在因为一个奇怪的巧合
seconds. Now by a strange coincidence, we live during
我们生活在地质学历史上一个地球
a time in geologic history when the Earth’s closest
距太阳最近的时期,几乎与一年两次的
approach to the Sun happens almost perfectly to
地球的斜轴正对太阳的那其中一次同时发生。
coincide with one of the two times of year when the
所以这多加的两天
Earth’s tilt is oriented directly towards the Sun. So these
的影响增加了。
two day lengthening effects add up. And on December
而且在12月22日,用日晷测量的一个太阳日的长度
22nd, the length of a solar day as measured by a sundial
会是一年最长。86430秒,
will be the longest it is all year. 86430 seconds, for a
总计多了30秒。
grand total of 30 extra seconds.
哦,以免你想知道12月22日
Oh, and in case you’re wondering where those 30 extra
这多出的30秒去了哪里,我就告诉你
extra seconds on December 22nd go, well they get
它们被推到12月23日,然后23日多余的秒数
pushed into December 23rd, and the extra seconds from
推到了24日,以此类推
the 23rd get pushed into the 24th, and so on which is
这是为什么太阳正午,或太阳正对头顶的时间
why solar noon, or the time which the Sun is directly
每天至点偏移约30秒的原因
overhead, shifts about 30 seconds later every day
这个太阳时与钟表时不断增加的不一致
around the solstice. This increasing disparity between
也是为什么,北半球
solar time and clock time is also why, in the Northern
最早的日落在至点
Hemisphere, the earliest sunset happens a few weeks
之前几周,最晚的日出出现在
before the solstice, and the latest sunrise happens a
在几周之后
few weeks after.

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