新托福阅读材料：The Front End of the Earth

利用地球规律掌握观赏流星的技巧

Enjoy star gazing?Then this Moment of Science is for you!

On a previous show we mentioned how not all shooting stars(流星) have the same speed. Some disappear almost immediately, while others make a nice slow line across the sky.

If you enjoy watching the slow ones, there is a way of upping your chances of seeing them. Go out just after sunset.That's because the slowest meteors come in the evening and the fastest ones come in the morning, just before sunrise.

Can you guess why?The speeds of meteors we see are increased or decreased by the earth’s motion. If we (1)charge into them (2)head-on our speed is added to their speed. If, on the other hand, we are moving away from them, our speed is (3)subtracted from their speed.

Now imagine yourself standing at the north pole, facing the direction the earth is moving through space. You rise up magically, far enough away that you can see the whole planet rotating underneath you while you yourself remain stationary. Which direction is it turning? Counterclockwise. Where is the sunlight? From your perspective, it’s on the left side of the planet.

That means that the front of the planet, or the part that’s charging into oncoming meteors, is the place where morning is beginning for people on earth. The back of the planet, meaning the part that’s moving away from meteors, is where evening is beginning.

It may seem funny, but every morning when you look up into the sky, you are looking in the same direction as the planet is moving. In the evening you are looking the other way. And the speed of shooting stars confirms it!