Subject: Estimating distances based on the speed of light

Hi. I have been under the impression that astronomical distance 
measurements are gauged on the speed of light. I understand this 
in principle, but am not sure how it is actually done. The reason 
I am interested in this has to do with the underlying assumptions 
associated with using light as a measuring tool. Since the speed 
of light varies according to the medium through which it travels, 
is it assumed that the average density of space is essentially 
equivalent to a vacuum (or at least equally dense in all 
directions)? Since it would be impossible to determine distance 
based on a measurement that could vary from time to time or from 
one direction to another, there must be a lot of stuff I am not 
taking into account. Any help would be appreciated.
Thank you,
Barry Weissglass

Re: Estimating distances based on the speed of light