Name: 【TED-ed】令人驚奇的太陽運轉 (The Sun’s surprising movement across the sky - Gordon Williamson)
Uploaded: 2021-07-14T21:01:20.000Z
Duration: 4 min 23 s

Suppose you placed a camera at a fixed position, took a picture of the sky at the same time everyday for an entire year and overlaid all of the photos on top of each other.

假設你將一台相機放在固定位置朝天空拍照，每天在同一時間拍攝一整年，然後將每張照片疊在一起。

What would the sun look like in that combined image?

太陽在這組照片中會是什麼樣子？

一個固定的點？或是一個圓形的路徑？

Oddly enough, it makes this figure eight pattern, known as the Sun's analemma.

奇怪的是，太陽會形成這樣的 8 字形圖樣，被稱為太陽的「日行跡」。

The Earth's movement creates a few cycles.

地球的轉動會產生出一些軌道。

First of all, it rotates on its axis about once every 24 hours, producing sunrises and sunsets.

首先，地球每 24 小時沿著 “自轉軸” 旋轉一周，產生日出及日落的現象。

At the same time, it's making a much slower cycle, orbiting around the sun approximately every 365 days.

同時，地球循一個更緩慢的軌道環繞太陽，大約 365 天繞一周。

Relative to the plane of its orbit, the Earth doesn't spin with the North Pole pointing straight up.

相對於它的軌道平面 (黃道面)，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 度的固定傾斜角度。

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

它被稱為「轉軸傾角」或 「傾角」。

A 23-degree tilt may not seem important, but it's the main reason that we experience different seasons.

傾斜 23 度看起來似乎沒什麼，但這是我們之所以會感受到四季最主要的原因。

Because the axis remains tilted in the same direction while the Earth makes its annual orbit, there are long periods each year when the northern half of the planet remains tilted toward the Sun while the southern half is tilted away and vice versa, what we experience as summer and winter.

當地球公轉時，一整年之中轉軸傾角都朝著相同的方向。每年會有很長一段時間，北半球一直傾斜朝向太陽，而南半球就傾離太陽，反之亦然，讓我們能因此感受到夏天及冬天。

During summer in a given hemisphere, the Sun appears higher in the sky, making the days longer and warmer.

在時值夏天的那個半球，太陽在空中會看起來較高 白天較長，也較溫暖。

Once a year, the Sun's declination, the angle between the equator and the position on the Earth where the Sun appears directly overhead reaches its maximum.

每年一次，太陽的赤緯，也就是赤道面與太陽在正頭頂上時的地表位置之間的夾角會達到最大值。

This day is known as the summer solstice, the longest day of the year, and the one day where the Sun appears highest in the sky.

而這一天就是夏至，一年當中白天最長的一天，而這一天的太陽會出現在天空的最高點。

So the Earth's axial tilt partially explains why the Sun changes positions in the sky and the analemma's length represents the full 46.8 degrees of the sun's declination throughout the year.

所以地球的轉軸傾角能夠部分解釋為什麼太陽會在天空改變位置，而「日行跡的長度」 代表了太陽赤緯在一整年中上下 46.8 度的掁幅。

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

但是為什麼是 8 字形而不是直線呢？

This is due to another feature of the Earth's revolution, its orbital eccentricity.

這是由於地球公轉的另一項特性：軌道離心率的關係。地球環繞太陽的軌道是橢圓形與太陽的距離隨著位置而改變

The Earth's orbit around the Sun is an ellipse, with its distance to the Sun changing at various points.

地球環繞太陽的軌道是橢圓形的，且地球與太陽的距離隨著位置而改變。

The corresponding change in gravitational force causes the Earth to move fastest in January when it reaches its closest point to the Sun, the perihelion, and the slowest in July when it reaches its farthest point, the aphelion.

在地心引力相對改變之下，造成地球在一月時地球處於最接近太陽的點：近日點時移動最快，而在七月地球處於最遠離太陽的點：遠日點時移動最慢。

The Earth's eccentricity means that solar noon, the time when the Sun is highest in the sky, doesn't always occur at the same point in the day.

地球的離心率變化也意謂著每天的中午，也就是當太陽在天空中位置最高的時候，並不一定會在相同的時間點上。

So a sundial may be as much as sixteen minutes ahead or fourteen minutes behind a regular clock.

所以日晷可能會比一般鐘錶時間早 16 分鐘或晚 14 分鐘。

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

事實上，鐘錶時間以及日晷時間 一年中只有四次是相同的。

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

「日行跡的寬度」 代表這種偏差的程度 (均時差)。在人類歷史大多數時期

So how did people know the correct time years ago?

那麼多年前，人是如何知道正確時間的？

For most of human history, going by the Sun's position was close enough.

在人類歷史中的大多數時期，依照太陽的位置來推估時間就已經夠準確了。

But during the modern era, the difference between sundials and mechanical clocks became important.

但是從近代開始，日晷與機械鐘錶之間的不同變得極為重要。「

The equation of time, introduced by Ptolemy and later refined based on the work of Johannes Kepler, converts between apparent solar time and the mean time we've all come to rely on.

「均時差」的概念是由托勒密所提出，後來依據約翰．克普勒的研究做了改進，在「視太陽時」與我們 現在所用的「平均太陽時」之間做轉換。

Globes even used to have the analemma printed on them to allow people to determine the difference between clock time and solar time based on the day of the year.

以前的地球儀上甚至印著日行跡，讓人們得以區別一年當中某日的鐘錶時間與太陽時間的不同。

Just how the analemma appears depends upon where you are.

日行跡的樣子會依據你所在的位置而有所不同。根據你所在的緯度會以某個角度傾斜或在南半球時呈現上下顛倒

It will be tilted at an angle depending on your latitude or inverted if you're in the southern hemisphere.

根據你所在的緯度，它會以某個角度傾斜，或當你身處南半球時呈現上下顛倒的樣子。

And if you're on another planet, you might find something completely different.

而如果你身處其他的星球上，你可能會有完全不同的發現。

Depending on that planet's orbital eccentricity and axial tilt, the analemma might appear as a tear drop, oval, or even a straight line.

根據星球的軌道離心率和轉軸傾角，它的日行跡可能會看起來像是淚珠、橢圓形，或甚至一條直線。