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

翻译人员: Kuang Jijia 校对人员: Shanshan (Alice) Lin

Suppose you placed a camera at a fixed position,

假设你把相机放在一个固定位置，

took a picture of the sky

每天都在同一时间对着天空拍照，

at the same time everyday for an entire year

一年后

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

把所有照片按时间顺序重叠起来。

What would the sun look like in that combined image?

太阳的轨迹会是什么样子的呢？

A stationary dot?

一个固定的点？

A circular path?

一个圆圈？

Neither.

都不是。

Oddly enough, it makes this figure eight pattern,

让人吃惊的是， 太阳的轨迹形状是一个“8”

known as the Sun's analemma,

这也被称作日行迹。

but why?

但是为什么会如此呢？

The Earth's movement creates a few cycles.

地球的运动包含很多圈。

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

首先，地球每24小时 围绕地轴自转一圈，

producing sunrises and sunsets.

于是有了日出和日落。

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

同时，地球还在以较慢的速度 围绕太阳旋转，

orbiting around the sun approximately every 365 days.

大约365天绕太阳一周。

But there's a twist.

不过有一个不一致，

Relative to the plane of its orbit,

和地球公转的轨道平面相比，

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

地球的自转轴不是指向北极的，

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

相反，有23.4°的倾斜。

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

这也被称作“轴倾角”。

A 23-degree tilt may not seem important,

一个23度的倾斜本没什么大不了，

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

但正是这个倾斜， 使得我们有了一年四季，

Because the axis remains tilted in the same direction

因为在地球围绕太阳公转时，

while the Earth makes its annual orbit,

自转轴始终是往一个方向倾斜，

there are long periods each year

所以，每一年有段时间， 北半球白昼更长，

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

也就是北半球往太阳那边倾斜，

while the southern half is tilted away

而南半球向另一面倾斜。

and vice versa,

反之亦然。

what we experience as summer and winter.

这也是我们经历的冬天和夏天的原因。

During summer in a given hemisphere,

在某个半球的夏季，

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

太阳在天空中显得位置更高， 也使得白昼更长，更暖和。

Once a year, the Sun's declination,

一年中会有一天， 太阳在我们看来的倾斜度，

the angle between the equator

也就是赤道和

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

公转平面的角度

reaches its maximum.

达到了最大数值， 这时，太阳仿佛正当头顶，

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

这一天被称作“夏至”， 一年中最长的一天，

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

也是太阳看起来最高的一天。

So the Earth's axial tilt

所以地球的自转轴的倾斜

partially explains why the Sun changes positions in the sky

能够部分解释太阳 在天空中位置变化的原因，

and the analemma's length

日行迹的长度

represents the full 46.8 degrees of the sun's declination

也代表了太阳在一整年

throughout the year.

倾斜的46.8°的角度。

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

但是为什么是“8”字形， 而不是一条直线呢？

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

这是因为地球旋转的另一个特征，

its orbital eccentricity.

轨道偏心率。

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

地球围绕太阳旋转的轨道是一个椭圆，

with its distance to the Sun changing at various points.

所以与太阳的距离是不断变化的。

The corresponding change in gravitational force

相应的地球引力的变化

causes the Earth to move fastest in January

使地球在一月时公转更快，

when it reaches its closest point to the Sun,

也就是到达离太阳最近的点时，

the perihelion,

这个点被称作“近日点”，

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

到达离太阳最远的点时，公转最慢，

the aphelion.

这个点也被称作“远日点”。

The Earth's eccentricity means that solar noon,

地球的椭圆运动意味着，太阳的正午，

the time when the Sun is highest in the sky,

也就是太阳在天空中最高的时候，

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

并不是总是在每一天的固定时间。

So a sundial may be as much as sixteen minutes ahead

所以太阳时间和时钟时间相比， 可能会提前16分钟，

or fourteen minutes behind a regular clock.

或是推迟14分钟。

事实上，时钟的时间和 太阳时间一年只会重合四次。

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

日行迹的宽度正体现了这种偏离的程度。

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

So how did people know the correct time years ago?

那么，过去人们 是怎么判断正确的时间呢？

For most of human history,

在人类历史上，大多数时间

going by the Sun's position was close enough.

都是通过太阳的位置来判断时间的。

But during the modern era,

但是到了现代，

the difference between sundials and mechanical clocks became important.

日晷测量的时间和机械时钟的时间 之间的差异变得越发重要了。

The equation of time, introduced by Ptolemy

“均时差”概念，由托勒密发明

后来由约翰尼斯·开普勒进行的改良，

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.

包含了太阳时间和平均时间的差距， 是我们现在主要使用的时间概念。

曾经的地球仪上面还会印上日行迹，

Globes even used to have the analemma printed on them

to allow people to determine the difference

使人们能知道

钟表时间和太阳时间的差距。

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

日行迹的形状对于 各地的人来说不完全相同。

Just how the analemma appears depends upon where you are.

倾斜的角度取决于你在的纬度，

It will be tilted at an angle depending on your latitude

南半球和北半球的人 测出的形状还会是颠倒的。

or inverted if you're in the southern hemisphere.

And if you're on another planet,

you might find something completely different.

如果你在另一个星球，

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

可能还会发现形状完全不同。

the analemma might appear as a tear drop,

根据那个星球的离心率和 自转轴的倾斜角度，

oval,

日行迹可能是泪珠、椭圆

or even a straight line.

甚至是直线。