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In the Northern Hemisphere, December has the fewest hours of daylight and the most darkness
在北半球，12月是昼最短夜最长的时候
because at that time the tilt of the Earth's axis is pointing the Northern Hemisphere away from the Sun.
因为在那个时候，地球的倾斜方向偏向北半球，远离太阳。
However, as counter-intuitive as it might sound, December actually has the longest days of the year.
尽管听起来可能有点反直觉，但12月实际上是一年中白昼最长的。
Modern clocks, of course, think every day is 86,400 seconds long, but that just the average length of a solar day over the course of the year.
当然，现代时钟告诉我们一天有86400秒长，但这只是一年中太阳日的平均长度。
A solar day is what you actually measure with a sundial,
太阳日是你用日晷实际测量出的，
or equivalently it's the time it takes for a line of longitude on the Earth to rotate back to face the Sun again.
或者说，它是地球上同一条经线转，旋转一周再次面对太阳所花费的时间。
This actually requires slightly more than 360 degrees of rotation because the Earth isn't just sitting in space rotating,
地球实际上的自转角度略大于360度，因为地球不只是在太空中原地自转而已，
it's also moving around the Sun so it has to rotate roughly 361 degrees before the Sun comes back perfectly overhead a particular place on consecutive days.
它同时在围绕太阳公转，它要旋转大约361度，才能再次使太阳回到地球某一点的正上方。
If the Earth's orbit were perfectly circular and its axis were perfectly upright, that would be the end of the story.
如果地球公转的轨道是完全标准的圆形，地轴是完全垂直的，那就没有后面这些事了。
However, the Earth's orbit is elliptical, so sometimes the Earth is slightly closer to the Sun,
然而，地球的公转轨道是椭圆形的，所以有时地球离太阳稍微近一些，
and the peculiarities of gravity mean it moves faster when it's closer to the Sun, so it goes farther around the Sun in 24 hours
加上地心引力的影响，地球离太阳越近，移动越快，所以地球在靠近太阳的未知，24小时内移动的距离更远，
so the Earth has to rotate slightly farther before the Sun comes back right overhead. 0.033 degrees farther, to be precise.
所以在太阳回到地球某一点正上方之前，地球需要多转一些角度。准确地说，是0.033度。
More rotation takes more time, so when the Earth is closest to the Sun, the real sundail measured day length is lengthened by about eight seconds.
旋转更多的角度就需要更多的时间，所以当地球离太阳最近时，实际的日长会延长8秒。
Plus the Earth's axis is tilted, which is what gives rise to the seasons,
此外，地轴的倾斜除了造成四季变换之外，
but also means that at the time of year when the tilt points towards or away from the sun, narrower slices of longitude are aimed directly at the sun.
也让地轴指向或偏离阳光的时候，较少的经线被太阳直射。
So as the Earth moves in its orbit,
所以当地球沿轨道公转时，
it has to rotate slightly farther in order for a particular line of longitude to catch up with the changing direction to the Sun. 0.088 degrees farther, to be precise.
地球必须多转一些角度，才能跟上太阳的移动变化，准确地说，是多转0.088度。
And again more rotation takes more time,
还是那个道理，旋转更多的角度就需要更多的时间，
so when the Earth is tilted towards the Sun, the real sundial measured day length is lengthened by about 21 seconds.
所以当地轴指向太阳时，实际太阳日的长度要长大约21秒。
Now by a strange coincidence, we live during a time in geologic history when the Earth's closest approach to the Sun happens almost perfectly
机缘巧合，现在我们处在地质学史上一个特别的时间段，地球离太阳最近的时刻
to coincide with one of the two times of year when the Earth's tilt is oriented directly towards the Sun.
与地轴指向太阳的两次中的其中一次重合了。
So these two day lengthening effects add up.
所以这两个会使白日增长的事件效果叠加。
And on December 22nd, the length of a solar day as measured by a sundial will be the longest it is all year.
在12月22日这一天，日晷所测得的实际太阳日时长是一整年中最长的。
86,430 seconds, for a grand total of 30 extra seconds.
是86430秒，整整多了30秒。
Oh, and in case you're wondering where those 30 extra extra seconds on December 22nd go,
对了，可能你想知道12月22日多的那30秒去哪了，
well they get pushed into December 23rd, and the extra seconds from the 23rd get pushed into the 24th, and so on
这30秒被挪到了12月23日，然后23号多的30秒被挪到24号，以此类推
which is why solar noon, or the time which the Sun is directly overhead, shifts about 30 seconds later every day around the solstice.
这也就是为什么正午，也就是太阳在地球某点正上方的时刻，在冬至附近每天会延后30秒。
This increasing disparity between solar time and clock time
太阳时与中标时间之间日益增加的差别
is also why, in the Northern Hemisphere, the earliest sunset happens a few weeks before the solstice,
也就解释了，为什么在北半球，最早的日出出现在冬至日的前几周，
and the latest sunrise happens a few weeks after.
最晚的日落出现在冬至日的后几周。
