Things in space being “xxxx lightyears away”, therefore light from the object would take “xxxx years to reach us on earth”

Question

I don’t really understand it, could someone explain in basic terms?

Are we saying if a star is 120 million lightyears away, light from the star would take 120 million years to reach us? Meaning from the pov of time on earth, the light left the star when the earth was still in its Cretaceous period?

In: 292

Answer 1

For every-day understanding, yes. Light takes time to travel so we call a light year the distance light travels in a year. Proxima Centauri is 4.2 light years away, meaning the light we are seeing is 4.2 years old.

There’s more to it but it’s way over an eli5 level. It requires General Relativity and doesn’t really have a big effect until we start considering VERY distant objects. Like in the billions of light years away.

Side note: that “more to it” is why the observable universe has a radius of 46.5 BILLION lightyears. Without that complex stuff, it would suggest that the furthest objects released their light 46.5 billion years ago, before the big bang, but that is of course impossible.