What does Mass want to tell us by writing "At the edge of darkness"?

Mass 写《在黑暗的边缘》想告诉我们什么？

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At the edge of darkness, after about eight hours, I turned around and saw the Earth from our altitude at Hubble, 568 kilometers up.

在黑暗的边缘，大约8个小时后，我转过身来，从哈勃望远镜上看到了568公里高处的地球。

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He looked over the edge of darkness.

他向悬崖下无边的黑暗望去。

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She was at the edge of darkness or the edge of light, depending upon which way you were going.

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