How are you watching something that happened "21 million years ago" when watching a supernova

i was reading this article..
http://news.yahoo.com/young-nearby-super…

and some scientist on the video says that you're watching something that happened 21 million years ago because it's 21 million light years away. I thought a "light year" was just the name of the distance (like miles, meters, etc) and had nothing to do with time. or thats what i was told..

what is the principle behind a "light year"

A light year is indeed a measure of distance, not of time. But it is related to time in a simple way: It is the distance that light travels in a year.

This means that when you watch an object that's a lightyear away, it took the light that you observe exactly one year to reach your eye.

Thus, if the object is 21 million lightyears away, the light that hits your eye right now has been on its way for 21 million years, and thus is a window into the past.

It even works on smaller scales: Light from the sun takes about 8 minutes to reach the earth, so you don't actually see how the sun is now, you see how it was 8 minutes ago.

http://en.wikipedia.org/wiki/Light-year

Since light travels at a finite speed, it takes time for it to get anywhere. So if it something like a supernova happened 21 million years ago, in a galaxy far, far away, its taken that long for light (traveling at 186,000 per second) to reach us.

As I like to say - "The Universe is huge and the speed of light is slow".

Crunch the numbers yourself. Find out how many seconds there are in a year and multiply by 186,000. That's a the distance of a light year. Now multiply again by 21 million and that's how far (in miles) that galaxy is away from us. Works best if you know how to use scientific notation and how to multiply with exponents.

Everything that we see is light, without ANY light we would not be able to see anything. It takes time for light to reach our eyes, but since light travels 186,000 miles per hour that time is almost nonexistent on earth.