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Kepler's first law of planetary motion states that all planets move about the Sun in elliptical orbits having the Sun as one of the foca.
开普勒行星运动第一定律指出，所有行星都在椭圆轨道上围绕太阳运行，太阳是焦点之一。
But what does that actually mean?
但这实际上意味着什么呢？
Well an ellipse is a shape that resembles a kind of squashed circle.
椭圆是一种类似于被压扁的圆的形状。
Its foci are two points within the ellipse that describe its shape.
它的焦点是椭圆内描述其形状的两个点。
For any point on the ellipse, the sum of that points distances to the two foci is the same.
对于椭圆上的任何一点，这两个点到两个焦点的距离之和是相同的。
The further apart the foci are, the more squashed the ellipse is.
焦点距离越远，椭圆受到的挤压就越大。
If the foci become so close that they are only one focus, you just have a circle.
如果焦点变得接近，以至于它们只是一个焦点，那么就是一个圆圈。
In reality orbits are never perfectly circular.
实际上，轨道从来都不是完美的圆形。
But we do know that the Sun will always be one of the foci of the elliptical path of an orbit.
但我们知道，太阳永远是轨道椭圆轨道的焦点之一。
Knowing that the Sun is a focus of the planet's orbit can tell us a lot about the shape of that orbit.
知道太阳是行星轨道的焦点可以告诉我们许多关于轨道形状的信息。
Kepler tells us that orbits are ellipses, which are like circles with some added eccentricity.
开普勒告诉我们，轨道是椭圆，就像有一些偏心率的圆。
But what is eccentricity?
但什么是反常呢?
How do you figure it out?
你是怎么想出来的？
Eccentricity measures how flattened an ellipse is compared to a circle.
偏心度衡量椭圆与圆相比的扁平度。
We calculate it using this equation.
我们用这个方程式来计算它。
So what does that mean?
那么这意味着什么呢？
Well, a is the semi-major axis, or half the distance along the long axis of the ellipse.
好的，a是长半轴，或者说椭圆长轴的距离的一半。
And b is the semi-minor axis, or half the distance along the short axis of the ellipse.
b是短半轴，或沿椭圆短轴的距离的一半。
The equation is a way to compare these axes to describe how squashed the ellipse is.
这个方程是比较这些轴来描述椭圆扁度的一种方式。
An ellipse with zero eccentricity would just be a regular old circle.
一个偏心率为零的椭圆只是一个普通的圆。
As eccentricity increases, the ellipse gets flatter and flatter until it just looks like a line.
随着偏心率的增加，椭圆会变得越来越平，直到它看起来像一条线。
An orbit with an eccentricity greater than one is no longer an ellipse but a parabola if e is equal to one a hyperbola it e is greater than one.
偏心率大于1的轨道不再是椭圆而是抛物线，如果e等于1，则是双曲线，e大于1。
For example, the giveaway that Oumuamua, the first interstellar comet, was not from around here was that its eccentricity was 1.2.
例如，第一颗星际彗星Oumuamua不是来自这里的，它的偏心率是1.2。
The eccentricity of Earth's orbit is only 0.0167.
地球轨道的偏心率只有0.0167。
Kepler's third law states that the squares of the sidereal periods of revolution of the planets are directly proportional to the cubes of their mean distances from the Sun.
开普勒第三定律指出，行星自转周期的平方与它们与太阳的平均距离的平方成正比。
Which what does that mean?
这是什么意思？
Basically it's saying that how long a planet takes to go around the Sun, its period, is related to the mean of its distance from the Sun.
它的意思是说一颗行星绕太阳一周的时间，也就是它的周期，与它与太阳的平均距离有关。
That is the square of the period divided by the cube of the mean distance is equal to a constant.
周期的平方除以平均距离的立方等于一个常数。
For every planet, no matter its period or distance, that constant is the same number.
对于每一颗行星，无论它的周期或距离，这个常数都是相同的数字。
Kepler's second law tells us that a planet moves more slowly when it's further from the Sun.
开普勒第二定律告诉我们，离太阳越远的行星运动速度越慢。
But why should that be?
但为什么会这样呢？
Well, as a planet orbits the Sun it may not keep a constant speed but it does maintain its angular momentum.
当一颗行星围绕太阳运行时，它可能不会保持恒定的速度，但它的角动量会保持不变。
Angular momentum is equal to the mass of the planet times the distance of the planet to the Sun times the velocity of the planet.
角动量等于行星的质量乘以行星到太阳的距离乘以行星的速度。
Since the angular momentum doesn't change, when the distance increases the velocity has to decrease.
由于角动量不变，当距离增加时，速度就会减小。
That means when the planet gets further from the Sun it slows down.
这意味着当这颗行星离太阳更远时，它的速度会变慢。
Kepler's second law deals with the speed of planets orbiting the Sun.
开普勒第二定律研究的是行星绕太阳运行的速度。
So does it tell us at what point the Earth is moving at top speed?
那么它能告诉我们地球在哪个点上速度是最快的吗？
The second law tells us that the Earth moves fastest when it's closest to the Sun, or at its perihelion.
第二定律告诉我们，当地球最接近太阳时，或在近日点时，地球运动最快。
That happens in early January.
这种情况发生在1月初。
At that point Earth is about 92 million miles from the Sun.
那时，地球距离太阳约9200万英里。
Meanwhile it's at its slowest in early July, at its furthest point from the Sun, or aphelion.
同时，它在七月初是最慢的，距离太阳或远日点最远。
That greatest distance is about 95 million miles.
这一最大距离约为9500万英里。
That difference of 3 million miles might sound like a lot but Earth's orbit is so vast that it's actually merely circular.
300万英里的差距听起来可能很大，但地球的轨道非常大，以至于它实际上是接近圆形的。
