When we say a body in space is X light years away from us(say 5000), how do we know that it took 5000 years for this light to reach us?
And, how accurately can we measure this figure?
In: 5
So I’m going to assume that by “How old a light source is” you mean to ask “How old the *light that left the source is* because then the rest of your question makes sense.
I am nowhere close to an expert in astrophysics, but: To answer your first question I’ll consider the thing you said first.
>When we say a body in space is X light years away from us(say 5000)
So establishing this itself is the difficult part. The better we do this, the more accurately we can say how much time it took light to reach us. Basically our car goes at a fixed speed. If we want to know how long it takes to get from home to school we only need to measure the distance as well as we can.
In the case of heavenly bodies, astronomers use triangulation methods using data from multiple observation locations on Earth with the help of a cool trick called **parallax**.
I should also say, there are also relationships between how bright an object shines and how far away it must be – **apparent brightness** I think it is. Essentially if candle very light, very close if not, very far.
Once they’ve done the best they can with their fancy telescopes, they know the speed of light is just one number so they multiply that speed with the distance and boom, they get the amount of time the light has to have been traveling before it entered our eyes or telescopes here on Earth.
>And, how accurately can we measure this figure?
Well I’ll really have to defer to someone more informed on this one but my opinion is that at this point we are able to measure distances to far away objects very very well and of course the further away an object is, our abilities decrease. Someone better informed can probably comment further on the specifics.
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