The Sun’s surprising movement across the sky - Gordon Williamson

譯者: Pao YuChen 審譯者: Max Chern

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,

there are long periods each year

每年會有很長一段時間

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

北半球一直傾斜朝向太陽

while the southern half is tilted away

而南半球就傾離太陽

and vice versa,

反之亦然

what we experience as summer and winter.

我們因此感受到夏天及冬天

During summer in a given hemisphere,

時值夏天的那個半球

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

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,

一年一次，太陽的赤緯

the angle between the equator

也就是赤道面

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

與太陽在正頭頂上時的 地表位置之間的夾角

reaches its maximum.

達到最大值

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

這一天就是夏至， 一年當中白天最長的一天

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

這一天的太陽在天空的最高點

So the Earth's axial tilt

所以地球的轉軸傾角

partially explains why the Sun changes positions in the sky

部分解釋為什麼太陽 會在天空改變位置

and the analemma's length

而「日行跡的長度」

represents the full 46.8 degrees of the sun's declination

代表太陽赤緯在一整年中 上下掁幅為 46.8 度

throughout the year.

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,

這是由於地球公轉的另一項特性

its orbital eccentricity.

軌道離心率

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

地球環繞太陽的軌道是橢圓形

with its distance to the Sun changing at various points.

與太陽的距離隨著位置而改變

The corresponding change in gravitational force

在地心引力相對改變之下

causes the Earth to move fastest in January

造成地球在一月時移動最快

when it reaches its closest point to the Sun,

那時地球是在最接近太陽的點

the perihelion,

就是近日點

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

而在七月移動最慢 那時地球是在最遠離太陽的點

the aphelion.

就是遠日點

The Earth's eccentricity means that solar noon,

地球的離心率意謂著中午

the time when the Sun is highest in the sky,

也就是當太陽在天空最高的時候

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

每一天不一定會在相同的位置

So a sundial may be as much as sixteen minutes ahead

所以日晷可能會多到

or fourteen minutes behind a regular clock.

比一般鐘錶時間 早 16 分或晚 14 分

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）

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,

在人類歷史大多數時期

going by the Sun's position was close enough.

依照太陽的位置已經夠準確了

But during the modern era,

但是近代

the difference between sundials and mechanical clocks became important.

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

「均時差」的概念是由托勒密提出

and later refined based on the work of Johannes Kepler,

後來依據約翰．克普勒的研究做了改進

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

在「視太陽時」 與我們 現在所用的「平均太陽時」做轉換

Globes even used to have the analemma printed on them

以前地球儀甚至印著日行跡

to allow people to determine the difference

讓人們得以區別

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

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.

日行跡的樣子依據你所在的位置而不同

It will be tilted at an angle depending on your latitude

根據你所在的緯度會以某個角度傾斜

or inverted if you're in the southern hemisphere.

或在南半球時呈現上下顛倒

And if you're on another planet,

而如果你在其他的星球

you might find something completely different.

你可能會有完全不同的發現

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

根據星球的軌道離心率和轉軸傾角

the analemma might appear as a tear drop,

它的日行跡可能會看起來像是淚珠

oval,

橢圓形

or even a straight line.

或甚至一條直線

翻譯：Pao YuChen