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

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.

A light year gets it's name from the fact that is how far light can travels in one year, that distance is an astronomical number, for instance the closest star to the Earth (not counting the Sun) is Proxima Centauri, it is 39,900,000,000,000 km away (or 4.22 light years) closer to home this same speed limit is why it takes up to 30 minutes to communicate with the rovers on Mars and something like 8 hours when a current in transit the "New Horizons" probe reaches Pluto in a few years and this is still within the solar system.

So when you look at any star you are look at the light that actually left that body, in the case of Proxima Centauri 4.22 years ago if the object is further away then, the center of our Milky Way galaxy is 30,000 light years away, so when it started its journey our ancestors where still living in caves.

http://heasarc.nasa.gov/docs/cosmic/nearest_star_i...

http://en.wikipedia.org/wiki/Light_year

actually the "light year" has slowed down since it started. If you left New York to Boston at 60 MPH but by the time you got to Boston, you were going 1 MPH, you would say that those people in NY are so far away as it takes 190 hours to get there when in fact at the real normal speed it would only take 3 hours. This 21 million thing is all made up. Cosmologists have been sucking on too many moon rocks.

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.

Now something that is 21 million light years away, means that it takes 21 million years for the light to travel from point A (supernova) to point B (our eyes). That means the light of the image of something that happened 21 million light years ago, took 21 million years for that image to reach us.

A light year is how long it takes for light to travel from objects in space to us. so when he said the star is 21 million light years away, he meant that the light we see is what the star was shining off 21 million years ago. Its crazy to think of, but a lot of the stars we see now are so far away that they don't even exist anymore, the light is just still traveling to us.

21 million light years away- it takes 21 million years to get to us traveling at light speed.

people say its looking into the past since it takes forever to reach us but i think it is dumb since were looking into time of dinosaurs and crap lol.

Light year-a year's worth of travel going at the speed of light