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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 at the same time everyday for an entire year and overlayed all of the photos on top of each other.
每天都在同一时间对着天空拍照，一年后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? A circular path? Neither.
一个固定的点？一个圆圈？都不是。
Oddly enough, it makes this figure eight pattern, known as the Sun's analemma, but why?
让人吃惊的是，太阳的轨迹形状是一个“8”，这也被称作“地球仪8字曲线”，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, producing sunrises and sunsets.
首先，地球每24小时围绕地轴自转一圈，于是有了日出和日落。
At the same time, it's making a much slower cycle, orbiting around the sun approximately every 365 days. But there's a twist.
同时，地球还在以较慢的速度围绕太阳旋转，大约365天绕太阳一周。不过有一个不一致。
Relative to the plane of its orbit, the Earth doesn't spin with the North Pole pointing straight up.
和地球公转的轨道平面相比，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, but it's the main reason that we experience different seasons.
一个23度的倾斜本没什么大不了，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 while the Earth makes its annual orbit,
因为在地球围绕太阳公转时，自转轴始终是往一个方向倾斜，
there are long periods each year when the northern half of the planet remains tilted toward the Sun
所以每一年有段时间，北半球白昼更长，也就是北半球往太阳那边倾斜，
while the southern half is tilted away and vice versa, what we experience as summer and winter.
而南半球向另一面倾斜，反之亦然，这也是我们经历的冬天和夏天的原因。
During summer in a given hemisphere, the Sun appears higher in the sky, making the days longer and warmer.
在某个半球的夏季，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 and the position on the Earth where the Sun appears directly overhead reaches its maximum.
也就是赤道和公转平面的角度达到了最大数值，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 partially explains why the Sun changes positions in the sky
所以地球的自转轴的倾斜能够部分解释太阳在天空中位置变化的原因，
and the analemma's length represents the full 46.8 degrees of the sun's declination throughout the year.
地球仪的8字曲线的长度也代表了太阳在一整年倾斜的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, its orbital eccentricity.
这是因为地球旋转的另一个特征，轨道偏心率。
The Earth's orbit around the Sun is an ellipse, with its distance to the Sun changing at various points.
地球围绕太阳旋转的轨道是一个椭圆，所以与太阳的距离是不断变化的。
The corresponding change in gravitational force causes the Earth to move fastest in January
相应的地球引力的变化使地球在一月时公转更快，
when it reaches its closest point to the Sun, the perihelion,
也就是到达离太阳最近的点时，这个点被称作“近日点”，
and the slowest in July when it reaches its farthest point, the aphelion.
到达离太阳最远的点时，公转最慢，这个点也被称作“远日点”。
The Earth's eccentricity means that solar noon, the time when the Sun is highest in the sky,
地球的椭圆运动意味着，太阳的正午，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 or fourteen minutes behind a regular clock.
所以太阳时间和时钟时间相比，可能会提前16分钟，或是推迟14分钟。
In fact, clock time and Sun time only match four times a year.
事实上，时钟的时间和太阳时间一年只会重合四次。
The analemma's width represents the extent of this deviation.
8字曲线的宽度正体现了这种偏离的程度。
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, going by the Sun's position was close enough.
在人类历史上，大多数时间都是通过太阳的位置来判断时间的。
But during the modern era, the difference between sundials and mechanical clocks became important.
但是到了现代，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 and later refined based on the work of Johannes Kepler,
“均时差”概念，由托勒密发明，后来由约翰尼斯·开普勒进行改良，
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。
Globes even used to have the analemma printed on them
曾经的地球仪上面还会印上8字曲线，
to allow people to determine the difference between clock time and solar time based on the day of the year.
使人们能知道钟表时间和太阳时间的差距。
Just how the analemma appears depends upon where you are.
这个8字形状对于各地的人来说不完全相同。
It will be tilted at an angle depending on your latitude or inverted if you're in the southern hemisphere.
倾斜的角度取决于你在的纬度，南半球和北半球的人测出的形状还会是颠倒的。
And if you're on another planet, you might find something completely different.
如果你在另一个星球，可能还会发现形状完全不同。
Depending on that planet's orbital eccentricity and axial tilt,
根据那个星球的离心率和自转轴的倾斜角度，
the analemma might appear as a tear drop, oval, or even a straight line.
地球8字轨迹可能是泪珠、椭圆甚至是直线。
