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Ordoshsen t1_j4kck6e wrote

Reply to comment by IAmAPhysicsGuy in How do we know that we are in a certain place inside our galaxy? and how do we know how big it is just by looking at the cross section we are in? by friday_panda

> Your brain can calculate distance because of the parallax that you get from having two eyes.

This only works for stars that are very close to us, not for most of the stars we see.

> Or you can see that people look smaller when they are further away (assuming you are familiar with the average size of a person).

If you use average size you're going to have a large error. You need to know how large the star actually is to determine how far it is. You can't just assume all stars are the same on average because we know that's not the case. Also I assume you're actually talking about apparent and absolute magnitude here.

I'll just add that this can be part of the puzzle when using dynamic paralax, but it doesn't work as you have worded it.

> Imagine these same principles applying to different positions in orbit around the Sun throughout the year

Yes, parallax works like that. But you will see no change in brightness of a star on our orbit.

> type 1A supernovae, and red / blue shift of light

Both of these are used for measuring distances to other galaxies. You can tell how far a Ia supernova (standard candle) has occured. But if you happen to see one from our own galaxy, you just know there was one binary star there but generally we just look at a bunch of galaxies and hope to see one there. But you probably didn't even know there was a star there before so you can't use that to measure stars you actually knew about.

red blue shift can tell you distance only of galaxies outside our neighborhood. You can't use it for measuring distance of stars that are next to us.

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mojowind t1_j4kmzg3 wrote

The Gaia satellite is able to measure parallax with an unprecedented precision. Parallax is possible for the thousands that can be seen with the naked eye, and potentially any star in the Milky Way which we have a line of sight view.

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craigiest t1_j4lccoh wrote

Are you just nitpicking details or are you arguing that the answerer is wrong in principle and that we don’t now the size of the Galaxy or our place in it?

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Ordoshsen t1_j4pebpg wrote

We do know the size. And I was wrong with how well we can measure the parallax. But other than the parallax the explanation was still mostly incorrect as far as I can tell.

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