A Stationary Satellite, part 2
地球同步卫星（二）
A geostationary satellite orbitsthe planet at the same rate as the planet rotates, so the satellite is always over the same patch of ground.
同步卫星以行星旋转的速度绕着行星运动，所以卫星总是处在地面同一块地方的上空。
Cool! you say. I’d like to have one of those about twenty feet over my house. Sadly, geos only work easily at one distance: 22,500 miles up.Why is that?
你会说，好酷！我宁愿有一个离我们房子只有二十英尺的卫星。不幸地，同步卫星只能在同一个高度工作，22,500 米的高空。为什么？
Other satellites can have different orbits, can't they?
其他卫星有不同的轨道，他们有吗？
Yes, they can. But that’s because they can also move at different speeds. A geo doesn't have that option—by definition it must orbit at the same speed as the earth turns. And that speed determines how high up it must be!
是的，他们有。那是因为他们能以不同的速度运动，而同步卫星没有那样的选择——他必须与地球自转同速运动。运动速度决定了他必须在的高度。
A satellite has to move at a certain speed to balance out gravity; otherwise it won't stay in orbit. Now, a geo must move at the speed the earth turns. If you put it in a very low altitude, that speed wouldn’t be enough to balance gravity.
卫星必须以某一确定速度运动来平衡我们的重力；否则将不能保持在同一轨道上。现在，同步卫星必须以地球自转的速度同速运动。如果处于很低的高度，卫星的速度将不足以平衡重力。
Go higher, though, and gravity gets weaker. At 22,500 miles, gravity will be just weak enough so that a satellite moving at the speed of the earth's rotation will stay in orbit without help.
高度越高，重力越小。在22,500的高空，重力小到可以使卫星在没有帮助的情况下与地球同速运动，并且始终在同一轨道上。
Of course, if you wanted to have a geo at a different altitude, you could always attach rockets to it and have them push continuously. But that's a pretty expensive option. At 22,500 miles, nothing else is required.
当然，如果你想有一个在不同高度的同步卫星，可以把卫星附在火箭，让火箭不断推动卫星前进。但是，这是相当昂贵的选择。在22500的高空，其他什么东西都不需要。