How can astronomers tell how ‘old’ light is? In other words, how can they tell if they are looking at an object 10 light years away or 1009 light years?

Question

How can astronomers tell how ‘old’ light is? In other words, how can they tell if they are looking at an object 10 light years away or 1009 light years?

In: Physics

Answer 1

To open another line of thinking on the question, have the photons in the light been travelling at, well, the speed of light and so, to them, are they aging for 10 years or 1009 years or are they still in the same instant?