There have been many memorable events witnessed by millions of Americans watching tv.   Some, however, have really made an impact on viewers and they'll never forget what they saw.

Where were you when 9/11 happened?  That day tops the list of the twenty most powerful events in tv history, according to a recent Nielsen and Sony Electronics survey.  To compile the list,  researchers asked 1,077 adults if they watched the events live on tv, what were they doing when that event occurred and if they remembered talking about it with other people.  These are the other events that made the list:

2.  Hurricane Katrina

3.  O.J. Simpson verdict

4.  Challenger space shuttle disaster

5.  Death of Osama Bin Laden

6.  O. J. Simpson white Bronco chase

7.  Earthquake in Japan in 2011

8.  Columbine High School shootings in 1999

9.   BP oil spill

10.   Princess Diana's funeral

11.  Death of Whitney Houston

12.  Capture and exectution of Saddam Hussein

13.  President-elect Barack Obama's acceptance speech

14.  Prince William's marriage to Kate Middleton

15.  Assassination of President John F. Kennedy

16.  Oklahoma City bombing

17.  George W. Bush/Al Gore presidential election results in 2000

18.  L.A. riots in 1992

19.  Casey Anthony verdict

20.  Funeral of President John F. Kennedy