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Newsgroups: sci.astro,alt.sci.planetary,alt.astronomy,sci.geo.geology
From: (Henry Spencer)
Subject: Re: Accuracy
Date: Mon, 10 Apr 2000 23:06:29 GMT

In article <8cl4t3$bhq$>,
Steve Jarrett <> wrote:
>Have you ever wondered why Kilometers are used to express very large
>distances in the first place?  For example, instead of saying the distance
>of Pioneer 10 is 11 Billion KMs, why not say 11 TM (Terameters)?  I'm
>surprised that Megameters, Gigameters et al aren't used.  After all, we
>use centimeters, nanometers, millimeters...

Part of the problem is that the larger prefixes are latecomers, and many
existing practices were well-established before they appeared.  (Giga and
tera were new when I was a schoolkid in the 60s; peta and exa are rather
more recent, and most people wouldn't even recognize zetta and yotta,
although they're ten years old now.)  Part of it's because of leftover
mile-based habits.

The bigger prefixes actually mostly work out pretty well, although it'll
be a long time before people get used to them...

The megameter is reasonably convenient for satellites and moons and such.
The Earth's radius is about 6.5Mm; the Moon is about 400Mm away.

The gigameter is a reasonable scale for the inner solar system.  The AU
is about 150Gm.  The asteroids are a few hundred Gm out.

The terameter is kind of clumsy.  Even Pluto is only about 6Tm away.
Maybe when we're operating out in the Oort Cloud...

The petameter is convenient for interstellar work.  A light-year is about
10Pm.  Alpha Centauri is about 41Pm away; the "local group" of stars is a
few hundred Pm in radius.

The exameter fits the galaxy fairly well.  The distance from here to the
galactic center is about 250Em.

The zettameter is about right for mapping this corner of the universe.
The Andromeda Galaxy is about 21Zm away.

Finally, the yottameter is about as big a distance unit as we'll ever
need, unless some new discovery gets made.  The farthest objects in the
known universe are only 100Ym or so distant, if memory serves (although I
may be slightly out of date on that).
"Be careful not to step                 |  Henry Spencer
in the Microsoft."  -- John Denker      |      (aka

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