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
There’s a thing called the “cosmic distance ladder”, which states the different ways you can measure the distance of astronomical objects depending on how far away they are.
For nearby stars, you use parallax. This is the change in the apparent location of an object depending on the angle you view it from. If you hold your finger up in front of your nose, and you close one eye and then the other, your finger will look like it’s moving left and right. Measure the angle between those two positions, do a bit of fairly simple trigonometry, and you can work out how far away your finger is.
You can do the same thing with stars. Measure the change in angle due to th Earth’s movement around the Sun, do some trigonometry, you get the distance. The angle is very small so it requires precise measurement, but it’s doable.
For objects further away than that, there’s something called a standard candle. This is an object where we know fairly well how bright it should be based on various properties. By comparing that known brightness to how bright it appears, we can work out how far away it is.
And one more method is redshift. Redshift is the doppler effect applied to light. For the sake of keeping this comment short I’ll assume you know what the Doppler effect is, you can look it up on this sub if you don’t. Point is, objects that are moving away from us look more red than they normally would, and the size of this shift depends on how fast they’re moving away from us.
But, thanks to the expansion of the universe, the velocity with which a distant object is moving away from us is directly proportional to its distance. The further away it is, the faster it is. So if you can measure the redshift you can work out the distance.
There are other methods, but these are the basics, and the easiest methods to understand.
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