As geologists, we examine layers of sediment on the Earth' s surface to approximate the dates of past geologic time periods.
作为地质学家，我们研究地球表面的沉积层以估算过去地质时期的年代。
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The moon's gravity is a sixth as strong as the earth's, so Chang'e-4's experiment was a test: would the earth plants know which way was up on the moon?