Distance in Lightyears

How do they know?

I see data posted all around the internet and in Science publications...

The nearest star to our sunProxima Centauri, is 4.2 light-years away, meaning the light we see from the star takes a little over four years to reach us.

Well OK then. We know WHEN the light reaches us, because that is now (or when we recorded it).

But just how do we know how OLD it is? i.e. Just when was it emitted?  It does not come tagged with it's birthdate.

Some Genius tells us that because Proxima Centauri is 4.2 light years away, we swallow the bait and assume the light was born/emitted 4.2 years ago.

That aforementioned Genius knows it is 4.2 light years away, because some other Genius has told him the light was emitted 4.2 years ago.

I think we are in a circle jerk here.

 

All light we see today is light we see today. We do not know how old it is.  So we should not make assumptions that suit the needs of others.

 

 

Please answer these questions if you can...

 

How do we know how far away is  Proxima Centauri?

How do we know that light emitted by it is 4.2 years old?

Dang.