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

Yeah, so I think it’s hard to understand with light because light travels so fast that in our day-to-day life it seems instantaneous. With a [1 trillion frame per second camera, you can capture light traveling through space,](https://youtu.be/EtsXgODHMWk) but it’s not something we can perceive with our eyes/brains.

Although sound is physically different than light, I think it’s a helpful analogy; we can understand sound travel from our own experiences. One example is an echo: if you shout into a canyon, the sound travels into the canyon, bounces off the wall and comes back to you. The sound you are then hearing is you from a second in the past – you’re not currently shouting, but you hear the shout from past you. Another example is when you hear a jet in the east, and look up and don’t see the jet right away because it’s already in the west. The sound you heard is the sound the jet made a second earlier when it _was_ in the east, but that sound didn’t represent it’s _current_ location by the time you perceived it.

Of course light is much, much faster than sound, but it still moves through space over time.

So just like your echo is your sound from a second in the past, the light that reaches us from a star a million light years away is the star’s light from a million years ago.