SJAA Ephemeris March 2004 | SJAA Home | Contents | Previous | Next


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Dave North


“Especially in the large structures, you'll see things almost as if you were flying toward them in an airplane. ”



March is arguably the finest month for evening mooning. The orb is high in the sky through most of the first quarter cycle. The weather is usually improving. And sometimes the seeing will be pretty good (this generally improves even more in April).

But you knew that already.

There will be a very close approach to Mars later in the month an occultation for some of our Canadian friends, but just a brush for us.

Oh well. Seems all the good stuff has been happening somewhere else for the last few years.

So this month I'm going to expand on an idea I mentioned a while back: observing the poles.

First, the terminator acts weird at the poles. It seems to hardly move at all one day to the next.

Of course on average it's moving just as far in longitude as the zippy stuff at the equator, but this does not translate to anywhere near as many kilometers in displacement.

Consequently, you get a much more thorough view of any particular feature, and get to see it with far less dramatic change on the next night.

Another interesting point is one pole or the other will generally be tilted away from the Sun, leaving a terminator visible throughout the month. Even at what seems to be full Moon, you can generally find a terminator.

Because there's almost always low light, you get some of the sharpest looks at jagged peaks in profile near the poles, and in fact some of the large farside crater rims were named as mountain ranges on some maps.

The poles actually became newsworthy a few years back because of speculation (still unconfirmed) that there are significant and possibly usable water deposits in the permanently shaded crater floors.

Some of us were loony enough to try to watch them crash into it maybe we'd see the plume.


But there are certainly some handy aspects to a polar location. With permanent low light the ground temperature flux is less than anywhere else on the Moon.

It's possible to arrange to have partial shade with minimal difficulty, making it possible to incorporate cheap temperature control.

And you can run solar cells 24/7.

But enough theory. What about observing?

The overwhelming 'extreme' is foreshortening. You'll be looking at everything almost edge on. Combine that with light that only changes slowly and it means you can study shadow effect, silhouettes and elevations more easily than on any other part of the Moon.

But wait, there's more! You'll note that many craters and structures seem to have Really Weird Shapes.

This is related to the 3D-to-2D effect that makes so many planetary nebulae have odd shapes a round crater will look "long," but a typical hexagonal structure can turn into a square, and long structures can even curve like a hot dog.

Keep an eye out for this effect and you'll soon find some of your own.

Of course my own favorite effect is the 'edge-on-crater' thang that I've mentioned before. Especially in the large structures, you'll see things almost as if you were flying toward them in an airplane.

You can see not only peaks and breaks in the wall, but you'll get a fascinating look at the terraces that form as the interior of the crater walls collapse.

Oh, one last thing.

A real favorite of almost any lunie is the horns.

Horns are the very tips of the crescent the points that seem unnaturally distended.

At their very best, you get a 'high point' (a peak) stuck out beyond the end, an almost starlike point like the final period when the story is over.


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