Told You That
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-supernova-dazzles-scientists-044627084.html
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.
Whitney Houston & George Michael – If I Told You That