# How can you find the distance to a galaxy using right ascension and declination?

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.

• 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. Mar 20, 2021 at 16:16
• Given the redshift is 0.009136, how can we calculate the distance to the galaxy?
– andi
Mar 20, 2021 at 16:39

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!)