Stellar orbits around the milky way.

I was just thinking about how this is done. I have looked online and I can't find a none mathematical explanation.

If I am getting it right, we get the orbits by measuring the radial and tangential velocities of several stars relative to our sun.

I thought the tangential velocity was unobtainable. Wouldn't you have to wait a century or two to be able to measure the tangential velocity.

I am sure the answer is easy. I am just not getting it off of the explanation I was given.

Can anyone explain this better to me.. or correct if I am misinformed.
 

mgb_phys

Science Advisor
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Do you mean the orbit of the sun around our galaxy or the rotation of other galaxies?
You can measure the movement of the sun by the position change of nearby and distant stars but the rotation of distant galaxies is from spectroscopy and so only measures velocity toward or away from us.
 

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