How far the light can be shine?

You can see stars, can't you?

 

Any distance until it interacts with something.

 

We've observed objects as distant as about 13 billion light years. So, we know of light that has traveled that far. (For reference, 13 billion light years is a little more than 1.2*10^26 meters. That's 120 trillion trillion meters, which is about 75 billion trillion miles.)

 

How do the scientists conclude that the particular light we received is x years old?

 

light year rings...

i would imagine triangulation and knowing the speed of light would be an unrealistically simple way of doing it...

 

http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/970415c.html


Doesn't sound all that precise to me, parallax aside?

 

Inertia still applies for EM radition, so i belive it depends entirely upon the environment into which you are shining the radiation. Since the majority of space is nearly a perfect vaccum EM radiation will countinue along its original vector untill acted upon by an outside force ie... black hole, nebula, telescope ect. Someone please correct me if im wrong.

 

Oh right, well I wouldn't know :grumpy:

 

they use the supernovae as a reference because of how much light it emits, right?

 

And eyeballs: if a photon manages to travel anything like a billion trillion miles I'd rather catch it with my eye than a ccd.

 

Indirect methods. One way is to measure the redshift- how far a particular emission line has been skewed. The age of the universe is determined, in part, by the redshifted background radiation (the 4 K noise).

Another way, as russ mentioned, is using "standard candles". These are sources that we "know" emit a certain amount of light:

http://csep10.phys.utk.edu/astr162/lect/cosmology/cosmicd.html

Parallax measurements are good only for very close objects.