I think it's kind of crazy how we don't know if the stars we see in the sky are really there.  We know that they were there, but we don't know if they're still there because it takes the light they emit so long to reach us.  We're technically looking back in time when we view them and we're seeing what they looked like in some cases thousands of years ago or more.  They could have died out or gone supernova in the meantime and we honestly have no idea.  

Technically, at any given moment we don't actually know if the sun is still there.  We know it was there eight minutes ago because that's how long it takes for the light from the sun to reach us, but if something happened to the sun five minutes ago, we wouldn't know yet.  

The other crazy part about that whole looking back in time thing is, I read a factoid once that if you were far enough away from the Earth and had a powerful enough telescope to point at it, you could look at the Earth and see dinosaurs walking around on it.  Like right now.  Because you'd be seeing the light that came from here millions of years ago.

This is part of why I'm not sure aliens have ever visited us.  If they were studying us from a distance, they wouldn't necessarily see any signs of civilization.  I know, I know they supposedly helped build the pyramids and all that jazz, but why would they even come here to do that?  At the point they left their world they probably wouldn't have seen anything here other than single-celled organisms.  Or possibly cockroaches.  Hey, let's go help build those slimy insects some big stone monuments!  Oops, now that we got here there appear to be hairless monkeys everywhere.  Oh well, let's use our highly futuristic technology to help them build some big stone monuments.  And then we'll take off and disappear and those people, who already had a system of record-keeping, will make absolutely no notes of our arrival or departure.

Sounds plausible to me.