🚀 go-pugleaf

gt3716a@prism.gatech.EDU (Josh Freeman) writes:
>On a side note, how is red shift calculated?

It's not calculated.
It's measured.

Sorry - what one does is take a spectrum of the galaxy and then match
the positions (in wavelength) of the observed spectral lines to a
reference spectrum, and see what the shift is.  Then you get the redshift, 
z = (lambda - lambda_0) / lambda_0.  In the non-relativistic case
v = z*c.  (In my last post on this I wasn't clear; special relativity
comes into this equation, however the linear redshift-distance law
is good up to high z.  Sorry.  Once upon a time people were going to 
measure the deviation from linearity to get q_0 ....)

Measuring redshifts is the less fallible part of the Hubble Law;
measuring distances is a whole lot harder.  This is the source of
most of the Hubble constant wars.

The above should be taken with a grain of salt as I have never
actually measured a redshift in that fashion.