because at that time, the tilt of the Earth's axis is pointing the Northern Hemisphere away from the Sun.

因為在這個時候，地軸的傾斜使得北半球照到比較少面積的陽光。

However, as counterintuitive as it might sound,

聽來有點出人意表，

December actually has the longest days of the year.

但12月其實有最長的白天。

Modern clocks, of course, think everyday is 86400 seconds long,

理所當然地，現代的時鐘告訴我們每一天是八萬六千四百秒，

but that's just the average length of a solar day of the course of the year.

但其實那只是一年中所有太陽日的平均。

A solar day is when you actually measure with a sundial

一個太陽日是你實際上使用日晷測量，竿影旋轉一圈的時間，

or equivalently it's the time it takes for a line of a longitude of the Earth to rotate back to face the Sun again.

或者等同於地球上同一條經線，自轉後再次面向陽光所需的時間。

This actually requires slightly more than 360 degrees of rotation because the Earth isn't just sitting in space rotating.

實際上，地球自轉的角度是些微大於360度，因為地球不只是太空中原地自轉而已。

It's also moving around the Sun, so it has to rotate roughly 361 degrees

地球還會對太陽公轉，它必須自轉大約361度，

before the sun comes back perfectly overhead a particular place on consecutive days.

在太陽再次回到地球某一點的正上方之前。

If the Earth's orbit were perfectly circular and its axis were perfectly upright,

如果地球公轉的軌道是完完全全圓形，而且地軸是完全直立的，

that would be the end of the story.

那當然就沒有這個問題。

However, the Earth's orbit is elliptical, so sometimes the Earth is slightly closer to the Sun,

不管怎樣，地球的公轉軌道是橢圓的，所以有時候地球會比較靠近太陽，

and the peculiarities of gravity mean it moves faster when it's closer to the Sun,

加上地心引力的特性，地球越靠近太陽的時候，會動比較快，

so it goes farther around the Sun in 24 hours,

在靠近太陽的地方，地球在24小時之內跑比較遠，

so the Earth has to rotate slightly farther before the Sun comes back right overhead.

所以在太陽回到天頂之前，地球必須自轉多一些。

0.033 degrees farther, to be precise.

準確來說，多轉0.033度。

More rotation takes more time,

自轉比較多就需要花比較多時間，

so when the Earth is closest to the Sun, the real sundial measured day length is lengthened by about 8 seconds.

所以當地球最靠近太陽的時候，真正日晷測量到的日長會多8秒鐘。

Plus, the Earth's axis is tilted which is what gives rise to the seasons,

此外，地軸的傾斜除了造成四季的變換之外，

but also means at the time of the year, when the tilt points toward or away from the sun ,

也讓地軸指向或偏離陽光的時候，

narrower slices of longitude are aimed directly at the sun,

比較少條經線會被太陽直射，

so as the Earth moves in its orbit,

所以當地球沿著軌道運行的時候，

it has to rotate slightly farther in order for a particular line of longitude

地球必須自轉多一些，才能使一個特定的經線區間，

to catch up with the changing directions of the sun.

才能追上日源的變換。

0.088 degrees farther to be precise.

準確來說，多轉0.088度。

And again, more rotation takes more time,

再次強調，比較多自轉就需要花比較多時間，

so when the Earth is tilted towards the sun,

所以當地軸指向太陽時，

the real sundial day length is lengthened by about 21 seconds.

真正日晷測量到的日長會增長大約21秒。

Now, by a strange coincidence,

如今，有個很奇特的巧合，

we live during a time in geological history

我們生存的這個時代在地質學史上剛好會使

when the Earth's closest approach to the sun happens almost perfectly to coincide

地球與太陽最靠近的時刻與

with one of the two times of the year when the Earth's tilt is oriented directly towards the Sun.

兩次地軸指向陽光的其中一次相重合了。

So these two day-lengthening effects add up

所以兩個使白天加長的效果加起來，

and on December 22nd, the length of a solar day as measured by a sundial

在12月22日這一天，日晷所測量到的太陽日

will be the longest day all year!

是整年最長的一天!

86430 seconds, for a grand total of 30 extra seconds.

86430秒，總共多了30秒。

Oh, and in case you're wondering where those extra 30 seconds on December 22nd go

喔對了，萬一你想知道12月22號那多的30秒跑到哪裡去了，

Well, they get pushed into December 23rd,

這30秒被到12月23號裡，

and the extra seconds from the 23rd pushed into the 24th and so on

然後在23號多的30秒就會被算到24號裡，

which is why solar noon or the time that the sun is directly overhead

這也是為什麼冬至正午或太陽直在天頂的時刻，

shifts about 30 seconds everyday around the solstice.

在冬至附近每天會延後30秒。

This increasing disparity between solar time and clock time

這太陽時和鐘錶時間逐漸增加的差異，

is also why in the Northern Hemisphere

也是為什麼在北半球，

the earliest sunset happens a few weeks before the solstice

最早的日出出現在冬至的前幾週，

and the latest sunrise happens a few weeks after.

最晚的日落出現在冬至的幾週後。