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  • Suppose you placed a camera at a fixed position,

    假設你將一台相機放在固定位置

  • took a picture of the sky

    朝天空拍照

  • at the same time everyday for an entire year

    每天在同一時間拍攝一整年

  • and overlayed all of the photos on top of each other.

    然後將每張照片疊在一起

  • What would the sun look like in that combined image?

    太陽在這組合照片中會是什麼樣子?

  • A stationary dot?

    一個固定的點?

  • A circular path?

    一個圓形的路徑?

  • Neither.

    都不是

  • Oddly enough, it makes this figure eight pattern,

    奇怪的是,太陽會形成這樣的 8 字形圖樣

  • known as the Sun's analemma,

    稱為太陽的「日行跡」

  • but why?

    但是為何如此呢?

  • The Earth's movement creates a few cycles.

    地球的轉動產生出一些軌道

  • First of all, it rotates on its axis about once every 24 hours,

    首先,地球每 24 小時 沿著 “自轉軸” 旋轉一周

  • producing sunrises and sunsets.

    產生日出及日落的現象

  • At the same time, it's making a much slower cycle,

    同時,地球循一個更緩慢的軌道

  • orbiting around the sun approximately every 365 days.

    環繞太陽,大約 365 天繞一周

  • But there's a twist.

    但是這裡有個驚奇的變化

  • Relative to the plane of its orbit,

    相對於它的軌道平面(黃道面)

  • the Earth doesn't spin with the North Pole pointing straight up.

    地球並非以北極朝上的方式旋轉

  • Instead, its axis has a constant tilt of 23.4 degrees.

    事實上,自轉軸有 23.4 度 固定傾斜的角度

  • This is known as the Earth's axial tilt, or obliquity.

    稱為「axial tilt/轉軸傾角」或 「Obilquity/傾角」

  • A 23-degree tilt may not seem important,

    傾斜 23 度似乎沒什麼

  • but it's the main reason that we experience different seasons.

    但這是我們感受到四季最主要的原因

  • Because the axis remains tilted in the same direction

    當地球公轉時,一整年之中 轉軸傾角都朝著相同的方向

  • while the Earth makes its annual orbit,

    每年會有很長一段時間

  • there are long periods each year

    北半球一直傾斜朝向太陽

  • when the northern half of the planet remains tilted toward the Sun

    而南半球就傾離太陽

  • while the southern half is tilted away

    反之亦然

  • and vice versa,

    我們因此感受到夏天及冬天

  • what we experience as summer and winter.

    時值夏天的那個半球

  • During summer in a given hemisphere,

    太陽在空中會看起來較高 白天較長,也較溫暖

  • the Sun appears higher in the sky, making the days longer and warmer.

    一年一次,太陽的赤緯

  • Once a year, the Sun's declination,

    也就是赤道面

  • the angle between the equator

    與太陽在正頭頂上時的 地表位置之間的夾角

  • and the position on the Earth where the Sun appears directly overhead

    達到最大值

  • reaches its maximum.

    這一天就是夏至, 一年當中白天最長的一天

  • This day is known as the summer solstice, the longest day of the year,

    這一天的太陽在天空的最高點

  • and the one day where the Sun appears highest in the sky.

    所以地球的轉軸傾角

  • So the Earth's axial tilt

    部分解釋為什麼太陽 會在天空改變位置

  • partially explains why the Sun changes positions in the sky

    而「日行跡的長度」

  • and the analemma's length

    代表太陽赤緯在一整年中 上下掁幅為 46.8 度

  • represents the full 46.8 degrees of the sun's declination

    但是為什麼是 8 字形而不是直線呢?

  • throughout the year.

    這是由於地球公轉的另一項特性

  • But why is it a figure eight and not just a straight line?

    軌道離心率

  • This is due to another feature of the Earth's revolution,

    地球環繞太陽的軌道是橢圓形

  • its orbital eccentricity.

    與太陽的距離隨著位置而改變

  • The Earth's orbit around the Sun is an ellipse,

    在地心引力相對改變之下

  • with its distance to the Sun changing at various points.

    造成地球在一月時移動最快

  • The corresponding change in gravitational force

    那時地球是在最接近太陽的點

  • causes the Earth to move fastest in January

    就是近日點

  • when it reaches its closest point to the Sun,

    而在七月移動最慢 那時地球是在最遠離太陽的點

  • the perihelion,

    就是遠日點

  • and the slowest in July when it reaches its farthest point,

    地球的離心率意謂著中午

  • the aphelion.

    也就是當太陽在天空最高的時候

  • The Earth's eccentricity means that solar noon,

    每一天不一定會在相同的位置

  • the time when the Sun is highest in the sky,

    所以日晷可能會多到

  • doesn't always occur at the same point in the day.

    比一般鐘錶時間 早 16 分或晚 14 分

  • So a sundial may be as much as sixteen minutes ahead

    事實上,鐘錶時間以及日晷時間 一年中只有四次是相同的

  • or fourteen minutes behind a regular clock.

    「日行跡的寬度」 代表這種偏差的程度(均時差)

  • In fact, clock time and Sun time only match four times a year.

    那麼多年前,人是如何知道正確時間的?

  • The analemma's width represents the extent of this deviation.

    在人類歷史大多數時期

  • So how did people know the correct time years ago?

    依照太陽的位置已經夠準確了

  • For most of human history,

    但是近代

  • going by the Sun's position was close enough.

    日晷與機械鐘錶之間的不同 變得極為重要

  • But during the modern era,

    「均時差」的概念是由托勒密提出

  • the difference between sundials and mechanical clocks became important.

    後來依據約翰.克普勒的研究做了改進

  • The equation of time, introduced by Ptolemy

    在「視太陽時」 與我們 現在所用的「平均太陽時」做轉換

  • and later refined based on the work of Johannes Kepler,

    以前地球儀甚至印著日行跡

  • converts between apparent solar time and the mean time we've all come to rely on.

    讓人們得以區別

  • Globes even used to have the analemma printed on them

    一年當中某日的鐘錶時間 與太陽時間的不同

  • to allow people to determine the difference

    日行跡的樣子依據你所在的位置而不同

  • between clock time and solar time based on the day of the year.

    根據你所在的緯度會以某個角度傾斜

  • Just how the analemma appears depends upon where you are.

    或在南半球時呈現上下顛倒

  • It will be tilted at an angle depending on your latitude

    而如果你在其他的星球

  • or inverted if you're in the southern hemisphere.

    你可能會有完全不同的發現

  • And if you're on another planet,

    根據星球的軌道離心率和轉軸傾角

  • you might find something completely different.

    它的日行跡可能會看起來像是淚珠

  • Depending on that planet's orbital eccentricity and axial tilt,

    橢圓形

  • the analemma might appear as a tear drop,

    或甚至一條直線

  • oval,

    翻譯:Pao YuChen

  • or even a straight line.

Suppose you placed a camera at a fixed position,

假設你將一台相機放在固定位置

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B1 中級 中文 美國腔 TED-Ed 太陽 地球 軌道 位置 傾斜

【TED-ed】令人驚奇的太陽運轉 (The Sun’s surprising movement across the sky - Gordon Williamson)

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    cci 發佈於 2016 年 02 月 08 日
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