英语时差：为什么最慢的流星出现在夜晚？（上）

On a previous show we mentioned how not all shooting stars 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 charge into them head-on our speed is added to their speed. If, on the other hand, we are moving away from them, our speed is subtracted from their speed.