18. Where did the moon come from?
How much do you know about the moon?Is it older or younger than the Earth? Where did it come from?Let me take you back to the earliest days of the Earth, four and a half billion (十亿) years ago. At that time, the Earth had no moon. It was moving around the Sun alone.
The early Earth was hit by rocks and comets (彗星). Today, there are no marks left from this huge ball. But to know more about the hitting, I've come to the Arizona desert to see a great hole in the ground. This is a beautiful, large and round hole, a near﹣perfect circle. It was formed when a shooting star fell down strongly into the Earth fifty thousand years ago. It's amazing how much damage that one passing rock can cause.
It must have been a mess with many rocks hitting the Earth. And then along came something much, much bigger. Another planet, the size of Mars, came into the path of the Earth. It was on the way to hit the Earth. It hit the Earth with a very fast speed. It was unbelievable and difficult to guess the power broken out by such a hitting!The hitting sent a large amount of running rocks into the path of the Earth. After the strong hitting, the broken pieces got together into a ball and the moon was formed, just 14, 000 miles away from the early Earth. This was the closest point it could have been. If it were any closer, the gravity (重力) would have pulled the broken pieces falling down strongly back to the Earth and there would be no moon.
Today the moon is just a rock without light but back then, it was a hot and round ball in space burning brightly. It must have looked amazing:a really large orange disc in the sky.
Imagine the scene:the first moonrise over the early Earth. Our world was no longer alone. It has a huge, powerful neighbour, and ever since then, the Earth has been a very different kind of planet.
1. When was the Earth moving around the Sun alone?
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2. What hitthe early Earth?
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3. How did the moon form after the strong hitting?
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4. What was the early moon like?
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5. What do you think of the very first moonrise?
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