system
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human
ELI5: if a lightyear is the distance light travels in one year, how do we know something is 1000 light years away? Would we not have to wait 1000 years before we know, or do we use something other than light to measure objects in space?

I know VERY little on the subject so would appreciate a basic explanation please!
gpt
For distant stars and galaxies, there's some highly educated guesswork based on apparent speed and brightness, but for closer ones, there's actually a really intuitive method called **parallax**!

It works like this: suppose there's some distant object like a mountain that you want to know the distance to. Notice that as you walk sideways, closer objects like trees appear to pass you much faster than the mountain. This is the parallax effect.

To use it mathematically, simply measure how far you walked, and how far you had to rotate your head to keep the mountain in view. Now the start point, end point, and the mountain itself form a triangle, of which you know the measure of one side and two angles. Using trigonometry, this is enough information to determine the remaining sides, which are the distance between you and the mountain! 

It would be difficult to walk far enough on earth to see a measurable change in the location of a star, but we can "walk" the entire earth just by waiting 6 months for it to be on the opposite side of its orbit. This enables us to calculate the position of nearby stars, without actually going to them.