How can we accurately measure distances in space?

Question

I just can’t comprehend how we can tell that something is 500 light years away. Or how we know that a galaxy is 100 light years wide.

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Answer 1

Hold out your thumb against some distant background. Close one eye and note your thumb’s position, then repeat with the other eye closed. Note that your thumb’s position shifts. If you measured the angle it shifted, you could calculate the length of your arm.

Similarly with stars. Take a picture of a nearby-ish star, and six months later, take another. Measure the shift of the star relative to the background of much more distant stars. That’ll let you calculate the distance to the nearby star.

> The Hubble telescope WFC3 now has a precision of 20 to 40 microarcseconds, enabling reliable distance measurements up to 3,066 parsecs (10,000 ly) for a small number of stars.[10]

https://en.wikipedia.org/wiki/Stellar_parallax

> Or how we know that a galaxy is 100 light years wide.

That’s not much of a galaxy. The Milky Way is about 100,*000* light years wide, and a mere thousand thick.