Mars and Earth have orbits which change with time.
火星和地球的轨道随着时间而发生改变。
So, orbits in general are ellipses.
所以，轨道总体上说是椭圆的。
但大部分轨道不会。
它绕着一颗红矮星运行。
Now, circular orbits are special.
圆形轨道非常特殊。
月球绕地球轨道运行。
What is the shape of their orbits?
转轨迹是什么形状的？
地球绕太阳轨道运行。
地球绕着太阳运行。
地球围绕太阳运行。
Let's pursue the idea of circular orbits.
我们用圆轨道的概念。
Orbits that cross, but maybe don't come close.
轨道交叉，但行星间也许不会太接近。
Some take only a few years to complete their orbits.
其中一些恒星仅需几年时间就能完成一周的运行。
As a planet orbits, it pulls a star back and forth.
当一颗行星绕它的轨道运行时，它能来回牵引恒星。
Orbits were not the sedate, orderly system they are today.
轨道还不是今天那样静止、有序的系统。
Can a planet be as large as the star that it orbits?
行星会和母恒星一样大吗？
The satellite orbits the earth in its regular orbit.
卫星在它的固定轨道里绕地球飞行。
In the past, information about orbits was a closely guarded secret.
在过去，关于轨道的信息是严格警戒的秘密。
The orbits around the Earth are too valuable to let this happen.
绕地球的轨道太有价值而不能让这些发生。
Their orbits bring them together with a minimum distance of about 200m.
他们轨道之间最小距离约200米。
Astronomers had thought that the orbits of the planets were predictable.
天文学家早就认为，几大行星的轨道是可以预测的。
That is the reason for the search for asteroids with appropriate orbits.
这就是为何要寻找轨道合适的小行星的原因。
However, this planet, in fact, orbits in what we call the habitable zone.
然而，事实上这颗行星的轨道在我们称之为的可居住的区域。
In these systems, a normal star orbits a massive yet invisible companion.
在这类系统中，一颗平常的星球围绕着一颗不可见的伙伴在运行。
It's always negative. Okay, later in the course we will cover elliptical orbits.
总是负的，好吧，以后的课，我们会讲椭圆轨道。
This classification is the analogy of our previous picture of stable orbits.
这种区分类似于前面的稳定轨道图形。
The general relationship between the closed orbits and the photodetachment cross section is given.
给出了闭合轨道和光剥离截面之间的一般关系。
The general relationship between the closed orbits and the photodetachment cross section is given.
给出了闭合轨道和光剥离截面之间的一般关系。
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