Re: _A Fall of Moondust_: goofs?

Here, Tim McDaniel <tmcd@xxxxxxxxx> wrote:
I Googled a little about Arthur C. Clarke's _A Fall of Moondust_, but
since I'm on dialup and my patience is limited ...

Our James covered this book on May 31, 2000 ("The Worldbuilders 9: _A
Fall of Moondust_, Arthur C. Clarke",
<8h44nr$nk1$1@xxxxxxxxxxxxxxxxxxxxx>). He mentioned that "We know now
that lunar dust doesn't act like this" without mentioning a reason.
Other articles mention vacuum welding. But I've seen elsewhere a
debunking (by Henry Spencer?) of the myth of All-Powerful Vacuum
Welding. So why *doesn't* moondust act like that?

What I remember (and it's probably also a Henry Spencer post) is
that the Lunar surface gets pounded by meteors all the time, big ones
and little ones. This does a great job of packing down the lower
layers of the dust. Also, lunar dust is tiny shards -- not rounded
grains like we get here at the bottom of the weather hole. So it's
high-friction, and when it's packed down it's essentially solid.

An easier-to-see goof: the _Selene_ starts to heat up because the dust
is a good insulator (and that's a good point), but the heat buildup
levels off because the dust starts to convect. But heat convection
only happens because the heat makes the substance change density and
therefore rise or sink (in gravity). I would not expect the dust,
being composed of sizeable solid particles, to have a noticable change
in density when heated.

Why not? Solids change size when heated. If you had a pool of rounded,
low-friction grains, I'd expect convection on a par with a liquid of
equivalent internal friction.

And if it flowed easily enough to convect enough to take the heat off,
I don't see why they would expect it to take "millions of years" for
the buoyant _Selene_ to bob to the surface again.

That is true.

I'll also mention chapter 10 (four pages in), which has

No, Tom corrected himself, this is the Moon, not Earth; here the
sun rises in the west.

My thought was "WTF?". Perhaps my Googledo has bad form, but I found
it oddly hard to find references to east and west on the Moon. But
eventually I hit <http://simkin.asu.edu/clem/>, "Clementine Moon
Maps":

One challenge for the beginner is that there are several
coordinate systems in use for locating features on the moon. The
image above, from JPL's Planetary Image Atlas goes from 0 to 360
degrees increasing to the East, or right. But the Clementine Lunar
Image Browser goes from 0 to 360 degrees increasing to the West,
or left. Other maps may go from -180 to 180, or 180 West to 180
East. The only longitudes common to all of these systems are zero
at the center of the nearside and 180 is the center of the
farside.

Many planetary data sets use positive longitude west coordinates,
although positive longitude east coordinates are more common in
recent work. Make sure you check the coordinate system used by
whatever resource you choose.

This implies that different maps disagree on the sign of the longitude
coordinate. But I *don't* expect them to disagree on which way west
is! As long as we draw maps from a notional "high above" viewpoint,
and we agree on which way north is -- which, for the Moon, we do --
then north-east-south-west will be top-right-bottom-left.

(Rotate that if you don't like north on top, but you won't flip it.)

Now, as to the sun rising in the west: I'm looking at my little
diagram, and I think it's just a goof.

--Z

--
"And Aholibamah bare Jeush, and Jaalam, and Korah: these were the borogoves..."
*
If the Bush administration hasn't subjected you to searches without a warrant,
it's for one reason: they don't feel like it. Not because you're an American.
.

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