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一言难尽的地日运动 – 译学馆
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一言难尽的地日运动

Earth's motion around the Sun, not as simple as I thought

A year on earth is measured by one complete trip around the Sun.
地球围绕太阳完整地旋转一周即为一个地球年
Seems simple enough,
这看似很简单
but there is a problem.
不过却有个问题
The earth doesn’t travel on a path around the Sun that returns to its starting point.
地球围绕太阳旋转一圈后 并不会返回它的起点
So how do we know when a year starts or ends?
那么 我们如何确定一年从什么时候开始或结束呢?
Well, one way called a sidereal year,
有一种叫做“恒星年”的方法
measures our orbit against the distant stars.
是测量地球相对于遥远恒星的运行周期来计算一年的时长
As viewed from the earth,
从地球上看
our orbit causes the Sun to appear to move
由于地球运转轨道的原因 太阳看起来
through the constellations of the zodiac on a path called the ecliptic.
像是沿着黄道在黄道十二宫中穿行
And when the Sun returns to its starting point,
当太阳重新回到起点时
a sidereal year has passed.
即为一个恒星年
This motion is difficult to observe directly
这种运动难以直接观测
because the Stars cannot be seen when the Sun is in the sky.
因为太阳当空时人们是看不到星星的
However, if you look at the sky before each dawn,
然而 如果你在每个黎明前仰望天空
the annual motion is very noticeable.
那么 太阳的周年运动是非常明显的
The last stars seen to rise are not always the same.
最后升起的星星并不总是相同
And within a week or two, an upward shift can be noted.
而且 在一或两周内可以注意到星星整体向上偏移
As an example, in July, in the northern hemisphere,
举个例子 在七月的北半球
Orion cannot be seen in the dawn sky,
你无法在黎明的天空中看到猎户座
but in August it becomes easily visible.
但到了八月 它就很容易被观测到
Measuring a year this way gives a period that is 365 days 6 hours 9 minutes and 10 seconds long.
用这种方法测出的一年是365天6小时9分钟10秒
Another possibility is to measure the year against the passing of the seasons.
另一种可能的方法是借助季节的更替来测量一年的时间
Because of the tilt of the Earth on its axis,
由于地轴是倾斜的
the position of the Sun in the sky changes from day to day throughout the year.
太阳当空的位置全年里都在日复一日的变化之中
If we were to take a picture of the Sun at noon regularly throughout the year,
如果我们每天中午定期给太阳拍摄照片 那么一年后
we would see the Sun moving on this path called an analemma.
我们会看到太阳在沿一条叫 “日行迹”的轨迹运动
On the days in its orbit when the earth is at a maximum tilt towards or away from the Sun,
当地球在其轨道上最大程度地向太阳倾斜或远离太阳时
the length of the daylight is at a maximum or a minimum.
将会迎来最长或最短的白天
These days are called solstices,
这两天被称为夏至和冬至
and the Sun will be as the top level or bottom right of the analemma.
太阳则会正好处在日行迹的最高或最低点
On the days when the earth’s tilt is perfectly sideways to the Sun,
在地轴倾斜方向恰好侧着朝向太阳的时候
the day-night are equal in length.
昼夜时长是相等的
These are the equinoxes.
这就是“二分点”
And the Sun will be at this crossover position in the analemma.
且太阳会处于日行迹的交点位置
When the Sun goes from one vernal equinox to the next,
当太阳从一个春分点运行到下一个春分点
a tropical year has passed.
即为一个回归年
Measuring this way gives a year that is 365 days 5 hours 48 minutes and 46 seconds long.
以此方式测出的一年是365天5小时48分46秒
The length of a year on earth is affected
地球上一年的时间长度会受到
by several gradual and cyclical changes in its orbit and its tilt.
其轨道和倾角的几个渐进且周期性变化的影响
First, there is the precession of the Earth’s axis.
首先 地轴存在岁差运动
Over a period of about 26,000 years,
大约每过两万六千年
the Earth’s axis traces out a circle in the sky.
地轴就会在天空勾勒出一个圆形轨迹
One result of this is that the North Star changes over time.
这便导致北极星在时间的推移中发生变化
Right now the Earth’s axis points towards Polaris.
当前 地轴指向“勾陈一”
Five thousand years ago, the axis pointed to a star in the constellation Draco,
五千年前 地轴指向天龙座中的一颗恒星
and 12000 years ago, the brilliant star Vega was the pole star.
一万两千年前的北极星是明亮的织女星
And because of the 26000 year cycle,
由于26000年周期的存在
Vega will be the pole star again in 14000 years.
14000年后 织女星将再次成为北极星
The precession of the equinoxes is caused
造成二分点存在岁差的主要原因是
primarily by gravitational forces of the Sun and the moon acting on the earth.
地球受到太阳和月球的引力影响
While the axial tilt is a primary cause of seasons on the earth,
虽然地轴的倾斜是地球上季节更替的主要原因
the distance from the Sun which changes throughout the year because of the elliptical shape of the Earth’s orbit
但因地球轨道呈椭圆形状而使全年处于变化之中的地日距离
contribute to small bit of temperature variations throughout the year as well.
也给一年中的气温变动贡献了一份力
When the axis is aligned, so it points towards the Sun during perihelion,
当地轴在近日点指向太阳时
one hemisphere will have a greater difference between the seasons,
其中一个半球上的季节变化会非常明显
while the other hemisphere will have milder seasons.
而另一半球的季节变化则会比较温和
The hemisphere which is in summer at perihelion will receive much of the corresponding entries in solar radiation,
位于近日点 处在夏季的半球会受到大量太阳辐射
but that same hemisphere will be in winter at aphelion
但该半球在远日点时将进入冬季
and have a colder winter.
且天气将更为寒冷
The other hemisphere will have a relatively warmer winter and cooler summer.
而另一半球则会有相对较温暖的冬季和较凉爽的夏季
When the Earth’s axis is aligned, such that aphelion and perihelion occurred near the equinoxes,
当地轴从远日点和近日点 到二分点附近时
the northern and southern hemisphere will have similar contrast in the seasons.
南北半球的季节差异将会大致相同
At present, perihelion occurs during the southern hemisphere‘s summer,
目前 南半球进入夏季时 地球位于近日点
and aphelion is reached during the southern winter.
而南半球进入冬季时 地球会到达远日点
So the southern hemisphere seasons
所以在其他因素相同的情况下
are somewhat more extreme than the northern hemisphere seasons when other factors are equal.
南半球比北半球的季节更极端一点
In addition, the gravitational effects of other planets
另外 由于其它行星的引力作用
caused the ellipse of our orbit to slowly spin around the Sun.
导致地球的椭圆轨道慢慢地绕着太阳旋转
It takes about 112000 years
每过11万2千年的时间
for the ellipse to revolve once relative to fixed stars.
参照遥远的恒星群 椭圆轨道会绕太阳旋转一个周期
When considered together, the two forms of precession at,
综合考虑时 这两种岁差运动将叠加
and it takes about 21,000 years for the Solstice to go from aphelion to aphelion.
这使至点两次出现在远日点位置的时间间隔约为21000年
The dates of the perihelion and the aphelion advance each year on this cycle,
在这段周期内 近日点和远日点到来的日期每年都会提前一点
an average of one day for 58 years.
平均每过58年 就会提前一天
The eccentricity of the Earth’s orbit is a measure of how round or how oval the orbits shape is.
地轨偏心率是用来衡量地球轨道的扁圆程度
Over thousands of years, the eccentricity of the Earth’s orbit varies,
几千年来 地轨偏心率一直处于变化之中
as a result of gravitational attractions among the planets,
原因在于行星之间的引力
primarily Jupiter and Saturn.
尤其是木星和土星
The orbital eccentricity cycles with a period of roughly 100,000 years.
地轨偏心率的循环周期大约为10万年
As the eccentricity of the orbit involves,
尽管地轨偏心率在变
the semi-major axis of the orbital ellipse remains unchanged,
但椭圆轨道的半长轴始终未变
so the length of the sidereal year remains unchanged.
因此 一个恒星年的时间长度也是恒定的
As the earth travels in its orbit,
当地球在轨道上运行时
the duration of seasons depends on the eccentricity of the orbit.
季节的持续时间取决于地轨的偏心率
When the orbital eccentricity is extreme,
当地轨偏心率非常大时
the seasons that occur on the far side of the orbit are substantially longer in duration.
处于轨道远日端的季节长度会更长
In addition to axial precession, there’s the axial tilt –
除了轴向岁差 还存在转轴倾角的问题
the angle the Earth’s rotational axis makes with its orbital plane.
它是地球自转轴相对其轨道平面的夹角
It is currently about 23.4° and is declining.
目前约为23.4° 并且正在减小
This tilt varies from 22.1° to 24.5°.
该夹角的变化范围在22.1°到24.5°之间
It makes one complete tilt and back every 41,000 years.
变化周期为41000年
This change in tilt is directly related to ice ages on earth.
这种倾角的变化与地球的冰河时期直接相关
The last maximum tilt occurred in 8,700 BC,
上一次最大倾角出现在公元前8700年
and the next minimum tilt will happen 11,800 AD.
下一次最小倾角将发生在公元11800年
The inclination of Earth’s orbit drifts up and down relative to the present orbit
地球轨道的倾角相对于当前轨道上下浮动
with the cycle having a period of about 70,000 years.
循环周期约为7万年
And the orbit also moves relative to the orbits of other planets as well.
而且地球轨道与其它行星轨道之间同样在发生相对运动
By calculating the plane of unchanged total angular momentum of the solar system,
通过对太阳系总角动量不变的平面进行计算
we can define the orbital plane called the invariable plane.
我们可以定义一个叫 “不变平面”的轨道面
It is approximately the orbital plane of Jupiter.
它与木星的轨道平面大致相同
The inclination of the Earth’s orbit has a 100,000 year cycle relative to the invariable plane.
相对于不变平面 地球轨道倾角变化周期为10万年
This 100,000 year cycle closely matches the 100,000 year pattern of ice ages.
这个10万年的周期与10万年的冰河周期非常吻合
A year on earth is directly determined by all the various orbital motions of the earth,
地球上的一年与地球各种不同的轨道运动直接相关
so if someone tells you how many years old they are,
所以当有人告诉你他们的“年”龄有大的时候
you might ask them, is that sidereal, tropical or anomalous years?
你也许要问一下 是恒星年 回归年 还是气候异常年?

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视频概述

何为恒星年?何为回归年?错综复杂的地日关系,引起四季更替,斗转星移。玄妙天机,在此揭秘!

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视频来源

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