How is the distance between Earth/Milky Way and another galaxy measured? This came up reading an article about identifing galaxies that are very far from Earth. I understand that the Doppler effect can be used to measure the speed a galaxy/star is moving in relation to Earth by calculations of the red shift of a stars spectrum. This got to question of how is it known that the distance star is the one moving vs is it the Earth that is moving. Then the question of how distance is actually measured.

1. Here is a decent article that summarizes some key points of the standard ways to measure these vast distances:
http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/970415c.html

2. BUT... and that's a BIG but:
Lookie here:
http://web.austin.utexas.edu/edcannon/betelgeuse.html
(Reputable websites citing Betelgeuse as anywhere from 300 to 1400 light years away!)

To me, this is evidence that we may or may not be as close as we think when citing distances of distant galaxies and even stars.

. This got to question of how is it known that the distant star is the one moving vs is it the Earth that is moving.


Or both that are moving?


But there is sometimes a way to tell. For example, astronomers know the rotational direction of stars around the milky way's hub and how they are distributed in what are called arms. Our sun is positioned near the inner rim the Orion Arm which is located a distance of 7.94±0.42 kpc from the Galactic Center and which includes millions of other stars orbiting in our same direction.

Knowledge of that motion provides astronomers with the key to understanding. An Orion Arm star overtaking the sun would register blue shifted-true. But since they know the direction in which the Orion arm is orbiting our galactic hub, then the approaching star "behind" is definitely the one closing the distance between us. If the sun were closing the distance then it would be going against at the flow.

If we were on a planet around that approaching star, then we would see the sun blue shifted but armed with that same knowledge we would tend to conclude that we are closing the distance because the sun is ahead of us in the Orion arm orbital direction.

It's like in the highway where traffic is going only one way. The car in front of us is assumed not to be backing up into us as we see it approaching when it slows down. It is we who are accelerating toward. Those in that car would assume that we are closing the distance since they are obviously going in the same direction we are.

The same applies to those stars moving in orbit around the hub but which are ahead of us. If they are blue shifted then we are the ones closing the distance not them. Those outdistancing the sun would appear red shifted. The sun overtaking a star would also register as blue shifted.

Others are red shifted as the sun leaves them behind due to its greater orbital velocity. Others moving at almost our same velocity would appear almost stationary with only very slight red or blue spectral shifts.

A more dramatic example: We see areas of the universe receding from us at near light speed. If indeed someone is looking in our direction from over there they would see us moving away from them at near light speed. But this is attributed not to the objects themselves but to the addition of or increase of space taking place between us caused by universal expansion. So neither are really moving at that speed by away via proper motion alone but are being separated as space itself expands at ever increasing speed-a speed thought not to be limited by the speed of light.

The blue shifted Andromeda galaxy is also a good example. We say it's moving in our general direct about 300 kilometers per second (186 miles/sec.) If we were over there we would see the Milky Way Galaxy moving toward us at the exact rate. But then again not all is confusion since Andromeda's angle of rotation is known and because of the individual stars proper motions can be inferred via their spectral shifts. The same principle is applied to any other light-emitting object.

OK OK, why is it that we as humans think we are so correct to think we have space/time figured out? That our measurements are correct and the devices we use give us the understanding we seek? We obviously know that our past has proven to be wrong about the stars we see, and why not the stars we see today? If it's based on what we see, could it be possible what we choose to see is wrong rather what is actually truth. So really what we want to say is of observer stand point or theory. So perhaps your answer may not be totally correct, right?

Johnjohn - scientists are always seeking to broaden man's understanding of nature, and as part of that work they develop ever more complete explanations of "how things work." As part of that it is inevitable that old ideas and theories are modified or expanded to better explain observations, or as sometimes happens the old ideas and theories get completely swept away as new models are developed that explain in a better way the observations we can make. That is the essence of the scientific method. No scientist worth his salt would ever say that we have it all "figured out" - if that was the case then there wouldn't be much point in scientists continuing to do their jobs. So yes - perhaps theories about astronomy 100 years from now will be radically different from what we think today, and that's OK, IF the new theories do a better job of explaining the observations we make than do the old.