How to measure the tangential component of velocity of receding starsor quasars?

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To measure the tangential component of velocity for receding stars or quasars, one must consider that redshift only provides the radial component of velocity. Nearby stars exhibit individual motion with both radial and tangential components, which can be measured by tracking their positions over time and applying trigonometry to calculate the angle of movement. In contrast, recession speed due to cosmic expansion, as described by General Relativity, is purely radial and lacks a tangential component. While distant receding objects may have some individual proper motion, it is typically negligible compared to their recession speed. Understanding these distinctions is crucial for accurate measurements in astrophysics.
aniketp
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Hi all,
How to measure the tangential component of velocity of receding starsor quasars?
I ask this because the velocity determined by observing the red shift will give only the radial component...
Thnx for replying...
 
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aniketp said:
Hi all,
How to measure the tangential component of velocity of receding starsor quasars?
I ask this because the velocity determined by observing the red shift will give only the radial component...
Thnx for replying...

nearby stars, like in our galaxy, are not receding from us as part of the general expansion. they just have their own individual motion, with radial and tangential components

tangential velocity of nearby stars in our galaxy can be measured by comparing positions over the course of many years
one figures out the distance to the thing, and one measures the tiny angle it moves per year, and uses trig. (multiply the distance times the angle in radians rate of change)

recession speed----that due to the expansion of distances a la General Relativity----is different from usual velocity. It only has a radial component. There is no tangential component to measure

a distant receding object can also have some individual proper motion. But this is often small compared with recession speed and may even be neglected.
 

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