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From: (Henry Spencer)
Subject: Re: astronomical distances
Date: Sat, 8 Aug 1998 06:58:27 GMT

In article <6qg6u9$>,
Frank Crary <fcrary@rintintin.Colorado.EDU> wrote:
>>> and why are both parsecs and light-years in common use?
>...The parsec is directly related to
>an observed quantity (the paralax of a star, as observed from Earth).
>That makes it analogous to the nautical mile.

Moreover, it makes it something that can be determined once and for all,
without any assumptions about conversion factors.  As noted in some
earlier discussion about the AU, often in the early days of measuring
something, we know the ratios of things like stellar distances much more
accurately than we know the conversion factor between the measurements and
normal units.  (In fact, this is still true of planetary masses.)  So it
makes sense to state results in accurately-known units, and once that's
been done for a while, it's hard to stop.

>In practice,
>professional astronomers tend to use parsecs exclusively. Light
>years are mainly used in popular science articles and science fiction...

I think a more accurate statement is that the use of the parsec is
confined to the traditional astronomy community -- although it is
near-universal there -- and as interstellar distances become a matter of
wider interest, the newcomers invariably use light-years.  For example,
space engineers studying interstellar travel almost always quote distances
in light-years.
Being the last man on the Moon is a |  Henry Spencer
very dubious honor. -- Gene Cernan  |      (aka

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