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  • In the Northern Hemisphere, December has the fewest hours of daylight and the most darkness

    北半球在一年之中,12月有最短的白日與最長的夜晚,

  • 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。

In the Northern Hemisphere, December has the fewest hours of daylight and the most darkness

北半球在一年之中,12月有最短的白日與最長的夜晚,

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【科學小常識】為何12月的一日最長?(Why December Has The Longest Days)

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    Michael Lu 發佈於 2015 年 12 月 28 日
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