= Spectroscopic velocity drift
{title2=$\dot v=c\dot z/(1+z)$}
= Velocity shift from redshift drift
{synonym}
For a transition with fixed rest wavelength, $\lambda_{\rm obs}=(1+z)\lambda_{\rm rest}$. The apparent spectroscopic velocity shift is $dv=c\,d\lambda_{\rm obs}/\lambda_{\rm obs}=c\,dz/(1+z)$. It is a convenient wavelength-shift unit, not necessarily a change in a source's <peculiar velocity>. Omitting the denominator $1+z$ overestimates the signal at high <redshift>.
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