On The Edge

Now and again a strong lunar libration will creep over the general observer's horizon and appear in the 'popular' press. Usually it's one of the 'easy' ones -- the sun is high over the limb.

More interesting to me, and harder to find, are those chunky librations where the terminator is near the edge.

Even more fun, they are usually best seen just before and after the full phase. What else is going on then anyway?

Traditionally my favorite target has been Mare Orientale. This month will be no exception.

On the evening of February 5, conditions may just be pretty hot. The terminator should be approaching the limb all evening, which makes for subtly changing light. There's a notable western libration shaping up, and a very strong tilt of the south toward us.

Since Orientale is more or less west-south-west, that might be a pretty good combination.

Might?

Okay, figuring this out with an acceptable degree of precision is something of a pain. And this time of year, it tends to be a little wasteful.

For one thing, if you think it's going to be a good shot, it's pretty darn easy to just take a peek that night. Even steady binoculars will tell you if it's worthwhile to set up a scope and go for it.

For another, the weather tends to shut us out, and all that careful research is down the drain.

But there is compensation: I've made similar observations through sucker holes while it's raining! You can get a pretty good look at Orientale even if you can't use much magnification, since it's huge.

Nudge to new scope owners: easy target if you have a map.

And of course there's a slight cool factor to observing in the rain (just set up under an overhang while the object is low enough to bring into view - but you knew that).

(You also know Mare Orientale means Eastern Sea, and that it's on the west side because the naming conventions were changed as part of the runup to the Apollo landings.)

I'd recommend this as an interesting effort for anyone who hasn't hunted near full.

But Orientale is hardly the only such target, and this is also a general call-to-arms - a suggestion that when you think there's nothing to do near full other than go to the SJAA meeting, you might also at least take a glance at the edges of the Big Light Bulb and see some things that might surprise you.

A couple of notable examples:

Pythagoras when the north limb is librated ... away! (Which it will be on the fifth, but maybe to better advantage on the 4th and 3rd. Take a look.) Under the right light, you can look at it almost edge-on, seeing the terraces in the crater walls almost as if you were looking at it from the opposing rim. A very different view.

When the tilt is just about opposite from the one we'll get (northeast turned toward us) you may catch sight of the terminator on Mare Humboldtianum. This is seldom mentioned in any observational guide and normally looks like a minor feature on maps or globes, but if you catch it in the right light you'll see something almost as surprising as Mare Orientale, and more accessible - it's better placed for us.

In theory, of course, you can see the terminator in similar 'on the edge' positions just before and after the new phase. But in reality, the sun is so close, the glare so bright, and the available observing time so short (before your target sets) that it's almost impossible to see any detail.

So your only real chance to nail this kind of thing is just before and after full. The few days trailing Bright Night are the best time to see Mare Crisium, the Gang Of Four and several other great features. And on the runup, you have the Aristarchus Plateau, Rumker, Rima Sirsalis, the huge Western Basins ... oh well. You get the idea.

Once again, I'm making the case for taking at least a glance at that time of month when you often think there's nothing to see at all. But while you're at it, keep in mind the librations.

To get a rough idea what's up, you can just consult your observer's guide. For a bit more comprehensive view, most astro software includes at least some basic information about the phases.

And of course there's the Hitchhikers Guide on shallowsky.com.

I'd give the complete URL, but I've been working on doing something a little silly this month.

I wanted to see if it's possible to write this column without (save for the title and last word) actually naming the Moon.

Nope.