<p>You can't use the RA and dec.  These tell you the direction that the galaxy is in, relative to the stars.  It is possible and normal for two galaxies to have the same (or very similar) RA and dec and be at completely different distance.</p>
<p>You can estimate the distance using the Hubble law <span class="math-container">$v= H_0 d$</span>, if <span class="math-container">$v$</span> is measured in km/s and d is in mega-parsecs the value of H_0 is approximately 70 (measuring this value accurately is a major problem in cosmology)</p>
<p>So <span class="math-container">$D= 2727/70 \approx 39$</span> So we estimate the distance to the galaxy to be 39 million parsecs  (multiply by 3.26 to convert to millions of lightyears).  This can only be an estimate, as the actual value of <span class="math-container">$H_0$</span> is not know with certainty, and the galaxy can have particular motion relative to the Hubble flow (for example, Andromeda is actually blue shifted, but it isn't a negative distance from us, as this value would suggest!)</p>

CC BY-SA 4.0
2021-03-21T13:58:09.987Z
So if the redshift was 0.009136, how can we use those values to calculate the distance to the galaxy?? Because I read in another answer that we can use RA, dec, and redshift to measure the distance to a galaxy.
andi
CC BY-SA 4.0
2021-03-21T16:07:45.927Z
You can convert redshift to resession velocity (v = cz is a good approximation for small values of z where relativity isn't needed.) and convert to distance by D = v/70.  This gives an estimate for the distance .  You don't use RA and dec at all.  These give direction not distance
James K
CC BY-SA 4.0
2021-03-21T16:10:07.070Z
For example, if I say "London is due north"  Can you tell me how far away it is?   No!  But if I also say "It takes me an hour to drive there"  Now you can estimate the distance of London from me. Because you have a model of how fast cars travel.  Similarly we have a model of how fast galaxies move away from us at different distances, so if we can measure this (using redshift) we can estimate their distance.
James K
CC BY-SA 4.0
2021-03-20T19:16:06.253Z
2021-03-20T19:16:06.253Z
James K
<p>For example, Galaxy NGC 3689 has the right ascension of 11:28:11 and declination of +25:39:40.0. Using these values, how can we find the distance to the galaxy? If it helps, its radial velocity is 2727 km/s.</p>

CC BY-SA 4.0
2021-03-20T16:16:02.723Z
It is not possible to do this with only RA and DEC, even if you have radial velocity. You would need the redshift, RA, and DEC to do this.
WarpPrime
CC BY-SA 4.0
2021-03-20T16:39:09.007Z
Given the redshift is 0.009136, how can we calculate the distance to the galaxy?
andi
<galaxy><distances><declination><right-ascension>
How can you find the distance to a galaxy using right ascension and declination?
andi
# How can you find the distance to a galaxy using right ascension and declination?

(Asked by: username_0 on 2021-03-20)

For example, Galaxy NGC 3689 has the right ascension of 11:28:11 and declination of +25:39:40.0. Using these values, how can we find the distance to the galaxy? If it helps, its radial velocity is 2727 km/s.

**Tags:** galaxy, distances, declination, right-ascension

> **Comments:**
> **username_1**: It is not possible to do this with only RA and DEC, even if you have radial velocity. You would need the redshift, RA, and DEC to do this.  
> **username_0**: Given the redshift is 0.009136, how can we calculate the distance to the galaxy?  

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## 1 answers

### Answer by username_2 on 2021-03-20. Score: 1

You can't use the RA and dec. These tell you the direction that the galaxy is in, relative to the stars. It is possible and normal for two galaxies to have the same (or very similar) RA and dec and be at completely different distance.

You can estimate the distance using the Hubble law $v= H\_0 d$, if $v$ is measured in km/s and d is in mega-parsecs the value of H\_0 is approximately 70 (measuring this value accurately is a major problem in cosmology)

So $D= 2727/70 \approx 39$ So we estimate the distance to the galaxy to be 39 million parsecs (multiply by 3.26 to convert to millions of lightyears). This can only be an estimate, as the actual value of $H\_0$ is not know with certainty, and the galaxy can have particular motion relative to the Hubble flow (for example, Andromeda is actually blue shifted, but it isn't a negative distance from us, as this value would suggest!)

> **Comments:**
> **username_0**: So if the redshift was 0.009136, how can we use those values to calculate the distance to the galaxy?? Because I read in another answer that we can use RA, dec, and redshift to measure the distance to a galaxy.  
> **username_2**: You can convert redshift to resession velocity (v = cz is a good approximation for small values of z where relativity isn't needed.) and convert to distance by D = v/70.  This gives an estimate for the distance .  You don't use RA and dec at all.  These give direction not distance  
> **username_2**: For example, if I say "London is due north"  Can you tell me how far away it is?   No!  But if I also say "It takes me an hour to drive there"  Now you can estimate the distance of London from me. Because you have a model of how fast cars travel.  Similarly we have a model of how fast galaxies move away from us at different distances, so if we can measure this (using redshift) we can estimate their distance.  

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