David North

High Moon, my favorite movie, continues this month. The moon reaches its greatest northern declination just a couple of days before first quarter, which means sunset and late first quarter viewing is still at the very best.

Of course, this is often coupled with some of the steadiest seeing in the San Jose area, so we get a double dose of opportunity.

Many of the targets in April will be very similar to those in February, save that the best window of opportunity has moved up a notch ... a few days earlier. That means the column from two months ago is very useful this month also.

But the very best day should be April 20.

Now, the difference between the best day and the second best, at early evening, isn't much (roughly a degree of elevation in April). So April 22-25 should also be as good, if not better, since the moon will linger longer at the top: when the moon reaches highest declination before first quarter, that means it's already sinking a bit when the sun sets! That's why March is, in some respects, the ideal month (except for weather.

Still, this month's day is the 20th, and we would do well to pay close attention. Why? Because of the slight but significant advantage it gives us in looking at the available goodies.

Most of the year, they are pretty low before first quarter, and the timing is clumsy near third quarter unless you like getting up in the middle of the night (which isn't most of us). And if you do, you'll still find the first quarter variation (light from the other side) to show a completely different moon, even with the same terminator placement!

So without further blather: the 20th.

It's a fairly static night in terms of terminator shift -- most observers will see less than a degree of movement in the light angle. This is both good and bad; good in that if something is showing well, you'll can give it a good look before conditions change -- but it's bad if you like watching the shift and play of light.

It's also a night of some anticipation, as the beginning of the "main sequence" of great moon sights is just over the horizon. But we're not concerned with them right now, since we have some very special showpieces.

The first and Greatest Hit for this night is the crater triptych Theophilus, Cyrillus (Rukl 46) and Catherina (Rukl 57). Even carefully held binoculars will easily show these three, so don't worry if equipment is somewhat modest.

Even at first glance it's clear they are of three different ages: Cyrillus has been pounded practically to oblivion, but Theophilus is nearly fresh... with Catherina somewhere between the two in time. But they are all of roughly the same size and form, so this area is literally a classroom in the look of craters from different eras.

Note in Theophilus how distinct the central peaks and the terraces on its seemingly sharp and cleanly seen walls. Contrast this with the fractured and ruined rim of Cyrillus and its eroded central peaks. Catharina, in some parts, is still fairly distinct, and has a somewhat cleaner floor (except where it has a pretty large crater superimposed).

But there is another interesting difference between Cyrillus and Catherina: the older crater also has a fairly prominent rille inside. It is usually the older craters that have inner rilles, and this also distinguishes it from its younger cousins.

Next, if we glance just a bit toward the terminator, we'll see the northern end of an extraordinary sight: a mountainous ring around Mare Nectaris, mostly right near the terminator (where it stands out best).

This will be one of the best views of the Altai Scarp you're likely to see (officially, Rupes Altai but that's just tiresome by comparison. Rukl 57) ... and indeed in the best conditions, people with sharp eyes can make it out with no optical aid at all.

It's a shock ring. Something really big whacked into the moon to make Mare Nectaris, and part of what was left after the dust settled was this shockwave ring frozen in the moon's crust for billions of years. And you thought your pimples never healed...

If you keep an eye on this area, you'll note that another, much fainter ring can be traced further out, when the light is just right. You'll also notice that quite a few curious formations on the moon (such as Vallis Rheita) are oriented radially from Nectaris. Whether they are strictly related is not certain, to the best of my knowledge.

There's plenty to look at and think about in this regard.

Of course, this is an excellent rille night if seeing permits. None of them are on anyone's Top Three Rilles On The Moon list, I don't think, but there are tons of them. I'll list the more notable in order from north to south, by Rukl page. You might enjoy them.

Rukl 5: Rima Sheepshanks. Great name. Long and a bit thin, but fun.

Rukl 14:

Rimae Burg, nearly a top five! For one thing, it's in Lacus Mortis (The Lake Of Death) which is one of my favorite grim names. For another, it's easy to get the main stem and hard as the dickens to get the faint extensions.

Rimae Danielli: Long and deep, an easy target usually.

Rimae Posidonius: Outstanding crater with tremendous interior structure. I've written way too much about this before, but it deserves the verbiage. Don't miss it.

Rukl 24: Rimae Plinius. Sometimes hard, sometimes simple. Sticks out of the end of the Montes Haemus at something called Promontorium Archerusia. Hmm.

Rukl 35:

Rimae Maclear and Sosigenes. I usually "glue" them together as they are very similar structures, both on the west side of Mare Tranquilitatis. Maclear is shorter and Sosigenes runs a loooong way. Some of Sosigenes will be in darkness, and the rest right on the terminator (which might show very well).

At the south end of Sosigenes you'll find Rimae Hypatia running off to the east. This is the easiest of the three technically, though the different orientations to the light mean sometimes one is easy and sometimes the other two. No telling.

Oh, one last note: sunset observers on the 18th will be able to see the crescent moon escaping from the Hyades... and perhaps an occultation or two.

David North; last updated: October 03, 2007 Prev Next