John Cage's 4'33

John Cage really did write a silent piece. I found out about it from the Horrible Histories Big Prom Party.
It's so funny! The orchestra doesn't play. They sit there quietly.
The conductor gives the downbeat and he uses a clock to time it.
You can hear all the coughs and you can see all the people looking at the orchestra.

Here's a recording of it:
http://youtu.be/zY7UK-6aaNA

I did find out that tacet means "It is silent".

I wonder how many bars each movement is?
What key signature is it written in?
Does the orchestra have to rehearse it?

Here's what John Cage says about 'silence'
http://youtu.be/pcHnL7aS64Y