Guard the Dark Sky

Look up at'the sky,how many twinkling little stars can you see?In fact,human eyes can see 2,500 stars.But becauseof brighter lights,such as city lights,the night is not dark any more2.In a city,you see fewer than 20 stars.抬头看看天空,你能看到多少颗闪烁的小星星?事实上,人的眼睛可以看到2500颗星星。但是因为有了更亮的灯光,比如城市的灯光,夜晚变得不再黑暗了。