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A geostationary satellite orbitsthe planet at the same rate asthe planet rotates, so the satellite is always over the same patchof ground.
同步衛星以行星旋轉的速度繞著行星運動，所以衛星總是處在地面同一塊地方的上空。
Cool! you say. I'd like to have one of those about twenty feet over my house.
你會說，好酷！我寧愿有一個離我們房子只有二十英尺的衛星。
Sadly, geos onlywork easily at one distance:
不幸地，同步衛星只能在同一個高度工作，
22,500 miles up.Why is that?
22,500 米的高空。為什么？
Other satellites can have different orbits, can't they?
其他衛星有不同的軌道，他們有嗎？
Yes, they can. But that's because they can also move at different speeds.
是的，他們有。那是因為他們能以不同的速度運動，
A geo doesn't have thatoption-by definition it must orbit at the same speed as the earth turns.
而同步衛星沒有那樣的選擇 他必須與地球自轉同速運動。
And that speeddetermines how high up it must be!
運動速度決定了他必須在的高度。
A satellite has to move at a certain speed to balance out gravity; otherwise it won't stay in orbit.
衛星必須以某一確定速度運動來平衡我們的重力；否則將不能保持在同一軌道上。
Now, a geo must move at the speed the earth turns.
現在，同步衛星必須以地球自轉的速度同速運動。
If you put it in a very low altitude, that speed wouldn't be enough to balance gravity.
如果處于很低的高度，衛星的速度將不足以平衡重力。
Go higher, though, and gravity gets weaker.
高度越高，重力越小。
At 22,500 miles, gravity will be just weak enough sothat a satellite moving at the speed of the earth's rotation will stay in orbit without help.
在22,500的高空，重力小到可以使衛星在沒有幫助的情況下與地球同速運動，并且始終在同一軌道上。
Of course, if you wanted to have a geo at a different altitude, you could always attach rockets toit and have them push continuously.
當然，如果你想有一個在不同高度的同步衛星，可以把衛星附在火箭，讓火箭不斷推動衛星前進。
But that's a pretty expensive option. At 22,500 miles,nothing else is required.
但是，這是相當昂貴的選擇。在22500的高空，其他什么東西都不需要。
