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Cast Your Shadow In The Sky

Dave North

We won't cast ours, which is why I won't be talking about the wonderful Lunar eclipse we won't be seeing on the 9th. But with any luck we'll see some nice pictures from other folks who will see it.

It's fun to see one at sunrise or sunset, though, because that's exactly what you are doing: casting your shadow on the Moon.

So if you ever get a chance at such a time, be sure to look.

Just to add insult to injury, however, that day has another distinction: it's also the day the Moon will be highest in the sky for January. The folks who do get to see it will see it very well indeed.

And that's also the beginning of my "Official Moon Season."

Why? When highest elevation happens between full Moon and first quarter, the Prime Time for evening viewing (just after Sunset to sometime before midnight) are when the moon is Way Up In The Sky. And that generally starts in January.

Throughout that timeslice, if you walk out and look at the Moon, you'll note it's pretty high in the sky most of the time during the evening.

That means, of course, less air in the way ... which means better seeing. Which means more detail. Which is the point.

So, since we can't look at the eclipse, what can we look at on this auspicious First Official Day Of Prime Mooning With Our New Christmas Telescope or maybe just A New Toy For Our Old Telescope?

Probably, the way these things go, rain.

But if that doesn't happen, you have to contend with that horror of knowitall amastronomers: the Wicked Nasty (Don't Look! It will BURN OUT YOUR EYES!!) Full Moon.

Of course it won't.

First point: there's no such thing as a full moon. Don't believe me? Take a look.

After the sun sets and the Moon starts coming up, right at the top there will be a terminator and a little dark slice... in fact, the only time there isn't a terminator is at eclipse, when there is a (very soft) terminator!

Unfortunately, the libration will be mostly in the off direction (though quite weak; only around a degree).

And if you look, you might see something pretty darn interesting.

Mare Humboldtianum. Up north. If we're lucky and I figured things right, the terminator should be running just about across the middle of it. In this kind of high relief, sometimes you can see not only the inner walls of this huge impact artifact, but also the first shock ring.

It's not quite as well-defined as the younger Mare Orientale of great repute, but it is much easier to see, and that counts for a lot.

If you do get a good look, you won't forget it.

At about 300 miles across, you're talking about one whopping impact. In good light, it's downright amazing, and the very best time to see it is not long after nearly full Moon.

If the seeing really screams, there is a weird porous, granular look to the floor that tells you what looks smooth sometimes isn't at all.

At other times of the year you can get more favorable librations (and so see more of it) but you'll not see it so high in the sky any other time.

It was named for a naturalist (Alexander von Humboldt) who wandered all over the earth, exploring new territory. When Johann Madler was casting about for a name (I don't know why he got the honor) he decided someone who explored the edges of the earth would be a good inspiration for the edge of the Moon.

One thing you won't see is how heavy the middle is: Lunar Prospector found a heavy mass concentration right at the center of the mare.

This phenomenon is not well understood, though many of the maria have similar "central gravity peaks." But the strength and centralized location of the Humboldtianum mascon was not well understood before Prospector.

So don't fall down when you visit there: you might not be able to get back up again!

Another thing you won't see is Belkovich, a bruiser of a crater that smacked into the northeast side of our featured mare. It has a great central mountain range, sawtooth jagged, that reminds me of the Sierras viewed from the eastern slope.

Why am I telling you this? Because, if you come back to the moon -just after full - over and over ... eventually you'll see it. And I think you'll get a kick out of it.

For tonight, just wander down the terminator and see what you can see. The edges of several other concealed maria should be just visible, along with some interesting craters (Humboldt, for example. What? Yeah, there's a Humboldt Sea and a Humboldt Crater, which is named after Alexander's brother Wilhelm, who was a Philologist. Whazzat? It means he read a lot about writing, then wrote about his reading. If that sounds loony to you, then maybe he should have a crater named after him....)

You also might enjoy Gauss, and aside from having a neat name, Endymion (near Mare Humboldtianum) is almost always pretty.

I'd like to hear your comments on what you see; send some email to and let me know where I went wrong (or right).

On the other hand, if it rains, go to and check out the scope I built this year.

It has nothing to do with anything, but what the heck, if you haven't anything better to do...

Mail to: Dave North
Copyright © 2001 San Jose Astronomical Association
Last updated: July 19, 2007

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