15．
In most situations，light helps us see．But nowadays humans are using so much that it is in fact a kind of pollution．
When it comes to looking at the night sky，too much light makes it （72）difficult for us to watch some of life's most wonderful views：stars，（73）planets and even galaxies（星系）．
According to scientific research，about 2，500stars can be seen by the human eye without （74）using any special equipment（设备）．But because of light pollution，you can only see 200to 300stars from today's countryside，and no more than ten stars from a city．In most big （75）cities，people cannot see the sky filled with stars like they did in their childhood．
Light pollution affects more than just our view of sky．Research （76）shows that lots of nighttime light can harm wildlife，too．
When birds fly to （77）another place over cities，they sometimes get lost by the brightness and fly in circles（圈） （78）untilthey drop from tiredness．Sea turtles need dark beaches for laying eggs，but they can't find their ideal places because of those bright lights．
Too much light at night （79）may even affect human health，but scientists are not sure of that．They are still learning more．
In order to prevent things from going （80）worse，governments and some organizations are working to reduce light pollution．Many cities and towns have taken action to reduce the use of lights at night．Lights are used only when and where they are （81）really needed．
Hopefully，in the near future，humans can enjoy the clear and beautiful sky again and all the wildlife will live a peaceful and undisturbed life，too．
