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Mooning

Highlights and Pielights

Dave North


On February 13, the moon will look as if it is nearly straight overhead. This is the "highlight" day of the month - the moon's greatest northern declination.

It will look almost full to the uninitiated, but those of us who've been watching it carefully will know it's there are still a few days yet.

The terminator will be running through an area that's not as popular as many - at first glance, it seems like there's nothing much near the terminator.

One of the more spectacular sites is now about a day old: the Aristarchus Plateau with it's incredible collection of rilles and the wonderful Schroter's Valley (Vallis Schroteri, for those who must).

The eastern section of Rima Sirsalis will be peeking out, but it will be better tomorrow.

But what's well placed tonight?

First, Galileo! This is an ideal night to find this unobtrusive little crater, named for the first person to publish descriptions of telescopic observations of the Moon.

It will be right about in the center of the disk, very near the terminator. It's visible in just about any scope with glass lenses, and won't look a much different from worst to best.

Why, you might ask, such a paltry crater for such a great name? Because of the influence of the Vatican. It was ... safer to name a small crater in an obscure part of the moon. Why take chances with burning at the stake?

Okay, that's a fun hors d'oeuvre, but what's for dinner?

Cheese.

To be precise, The Thin Cheese. One of the most interesting craters on the Moon, really.

If you let your gaze drift south past the monstrous Oceanus Procellarum, until you're at the terminator just a bit north of Clavius (you know Clavius, right?) you'll see a group of three large craters.

The largest is Schickard - almost the rival of Clavius, with obvious darkening in the floor caused by the infamous Mare Material, a basaltic lava.

Just south of it is the wonderfully apellated Phoclydes, about half it's size, but also with some faint traces of lava flooding.

Sandwiched between the two and slightly closer to the terminator is our main target: Wargentin.

Now there's a satisfying lava flow: it comes all the way up to the brim! It looks like a pie, or to the astronomish wags who nicknamed it, a thin round of cheese. Of course, they were playing on the folk humor that the moon is made of green cheese.

Never could figure out that one myself, since it isn't green...

So what happened? Best guess is this particular crater was formed over a weak spot, over a volcano waiting to happen.

When it did, the lava started flowing, and it flowed until it just barely filled up the crater ... no more, no less. One of those fascinating coincidences that we see so often.

The level of the "floor" of the crater is, in fact, about 1000 feet above the mean surrounding terrain!

If you look closely, you can see small wrinkle ridges and other deformations in the lava fill, but the real challenge is to see if you can find anyplace where the lava flowed over the edge, or any obvious sign of a rim that sticks above the interior flow.

Thin cheese indeed! It's Hot Rock Pie! Good though...

Wargentin? He was the director of the Stockholm Observatory in the late eighteenth century. Stockholm may not seem like the best place for astronomy, but it's not all that far from Tycho's observatory, place of some of the most important observations ever. Can't be all that bad.

Besides, he was also a statistician back when that was a newish game. Also, in the only picture I've seen of him, he wore a silly wig (at least, the artist stuck it on him).

I tried on a sillier wig over the new year's break, but I think it did more for me than that old white powdered thing did for him.

But enough of that.


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

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