长期以来，这种密度上的相似性促使人们猜测月球是从快速旋转的地球上分裂出来的，但这种想法创始于两次观测。
This similarity in density has long prompted speculation that the Moon split away from a rapidly rotating Earth, but this idea founders on two observations.
访问结束前，我被领到一个房间，在一个巨大的纯平显示器上有一个旋转的地球三维图像。
Toward the end of my tour, I was brought into a room where a three- dimensional image of the earth rotated on a large flat-panel monitor.
这些卫星处在平稳旋转的地球上空，几乎可以对全球的所有地区进行稳定的扫描，因此能很好地完成气象观察、通讯以及其它任务。
Such satellites—maintaining their steady sweeps as the earth revolves beneath them—cover all parts of the globe and hence are ideal for weather observation, communications and other tasks.
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