Sign up ×  I've done some search with the Nasa Extragalactic
Database (NED) and I have a very basic question about the
velocity/redshift conversion. For example, for the first object
of this page, we have $v=19791km/s$ and $z=0.066016$.  If we use
the simple formula $z=v/c$, we find the result of the database :
$19791/299792=0.0660157709345$. But now if we use the formula :
$z = \sqrt{\frac{c+v}{c-v}}-1$ we find $0.0683461749892$  Which
is the correct one and why ?  Thank you very much.
share|cite|improve this question I suspect that the velocity
formula must incorporate the expansion of the Universe. I'm not
sure though: I wouldn't expect it to be a ~3% effect at that
redshift. –  Warrick Jun 25 '12 at 11:29  2 Answers 2  up vote 2
down vote accepted  The simple formula is just the first-order
expansion of the more complicated one about $v = 0$, the latter
being exact for the Doppler effect of motion purely along the
line of sight. The $v$ here refers to the peculiar motion of the
galaxy.  Be aware that for