新托福听写训练 第19期（在线收听）

Listen to a talk in a geology class.

I often hear my friends say that the days pass much more quickly than they used to. But geologically speaking just opposite is true. In fact, a complete day which now lasts 24 hours actually used to be much shorter only about 18. How do we know that? Because of a number of

finally strata rocks like these, rocks form belong ancient shorelines almost a billion years ago. The tiny lines of these samples show us layers of light and dark formed by dust blown over the shoreline from the landsite. Alternating was the mud and sand deposited by the waves. So the space between one dark strata and next represents the time between one month high tide and the next. And very in fitness of layers show us the circle of the seasons as well. Together the data indicate there were fewer months for year way back them. That means that the moon was moving more slowly than as it revolved around the earth. So what caused the moon do speed up so much over the last billion years? It must be the tides, think about it. As the gravity of the moon pulls on the oceans to form the tides, all that the water is also pulling on the moon and with each rotation makes the moon move a tiny bit faster. And at the same time like the brakes on the wheel of the bicycle, the drag caused by the moon's gravity makes the earth turn just a tiny bit slower. And so after a billion years or so, one rotation meaning one day, takes a lot longer than it once did.