What is a Light Year?

Published March 12, 2014 by ashawnomy

We were watching a little bit of the Cosmos episode today in class and the idea of what a “Light Year” actually is came up.  I thought it might make a good post.  So, we’ll start with the basics:  a light year is the distance that light travels in one year.  Technically, this is about 6 trillion miles!  That’s far.  What this means is that if I sneezed on Earth and you were exactly 1 light year away, you wouldn’t hear me sneeze for exactly a year.  If you immediately said, “Bless You”, it would take another year before I heard you.  Therefore, if I sneezed and you “blessed me”, it would take 2 years before I heard it.  I wouldn’t know what you were talking about of course, because I sneezed 2 years before and probably wouldn’t remember.

Another way to look at this is to think that if a star is 100 light years away, the image we are seeing is 100 years old.  If that star were to suddenly fall into a black hole and completely disappear very quickly, we wouldn’t notice for 100 years, because that is how long it would take the light to reach us.

That’s pretty crazy.  Space is very big.  We are very small.


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