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From: gino@voder.UUCP (Gino Bloch)
Newsgroups: net.astro
Subject: Re: Measurement of Light Years and Distance
Message-ID: <479@voder.UUCP>
Date: Fri, 26-Oct-84 21:09:01 EST
Article-I.D.: voder.479
Posted: Fri Oct 26 21:09:01 1984
Date-Received: Sun, 28-Oct-84 06:48:00 EST
References: <872@ihuxp.UUCP>
Organization: National Semiconductor, Santa Clara
Lines: 23

> Can anyone tell me how the distance in light years is measured? 
> How do we know how far something is? Does the spectrum shift
> of light have something to do with it?
> 					F. C. McAtee

Not a trivial question.  In short: distances are measured by a long chain
of difficult measurements, deductions, and extrapolations, where the term
`noise-to-signal ratio' would be more appropriate than the usual term.

At greater length:  the first step is parallax, the apparent shift in position
of objects viewed from opposite sides of the earth's ORBIT.  This is good to
a few hundred light years with decreasing accuracy.  After that, knowledge
of such things as brightness as a function of spectral class and other
characteristics is used.  The latter leads to distance to nearer galaxies,
such as Andromeda (M31).  Properties of galaxies as a whole lead to estimates
for farther galaxies and clusters.  Finally, the red-shift observed in this
regime leads to extrapolation of distances to the farthest galaxies.

The above just scratches the surface to give a flavor of what goes on.
Read some good books on astronomy (I have no recommendations at my finger-
tips, sorry).  That will be fun, I think (but then I love astronomy).
-- 
Gene E. Bloch (...!nsc!voder!gino)