The Far Side of the Moon

1997 June 20

Akkana Peck

A favorable libration brought Mare Orientale, normally on the "far" side of the moon and invisible to earth, into view on June 20, 1997, and I got my first look at it from my driveway in Los Altos with the 6" Cave.

Seeing was somewhat shaky, and the view was better at 120x than at 181x. Grimaldi was visible in the finder as a prominent dark spot, like a miniature Mare Crisium, near the west limb of the moon, and southwest of Grimaldi, along the barely visible terminator (one day shy of full moon), Mare Orientale was visible in profile as a plateau surrounded by two concentric rings of mountain ranges (Montes Cordillera and Montes Rook).

Tracing the profile along the terminator was particularly interesting: the flatness of Orientale, then a steep drop to the valley to the south, then two rises where the concentric mountain ranges stood in relief against the unlit limb, and a pair of long dark streaks extending further northward, perhaps ejecta from the event which formed Orientale.

After exploring Orientale for a while, I couldn't resist swinging over to take a peek at Jupiter, still low in the southeast. All four Galilean moons were visible, with two very close to each other -- an occultation either about to occur, or having just occurred. I watched for a while, and within some fifteen minutes, the occultation occurred. In the 6" in ratty seeing, there wasn't much to see -- two points became one bright point. I hope to catch one of these events later in the summer when Jupiter is higher in the sky and more detail should be visible.

My Jupiter java applet revealed that (1) the two moons involved were Europa and Ganymede, and (2) the Date class time zone bug is biting me, now, too (the applet showed the occultation as taking place at 2:30 PDT instead of at 1:30 when it actually occurred). I really need to find a workaround for that.

Having satisfied my curiosity about the Jovian moon occultation, I swung back to earth's moon for one last look. The terminator had moved significantly while I was watching Jupiter, and now two craters which I had not noticed before stood out prominently off the north edge of Orientale, both with huge central mountains nearly filling the crater ("central peak" doesn't do justice to these mountains). Rukl is somewhat difficult to read for "far side" features, but I might guess that the crater to the southwest of Grimaldi and inside the concentric rings of mountains may have been Kopff; the one to the northwest could have been Schluter. I fought off the urge to sleep for a little while longer, long enough to make a sketch of this area which I may not have another chance to see for quite a long time (until the next such favorable libration).

I have added my comments on Mare Orientale to the Hitchhiker's Guide to the Moon. Please use this resource and help contribute to it, so we can all share knowledge of interesting things to look for on the moon! (Thanks to loonie extrordinaire Dave North for showing me Orientale in the first place -- I wouldn't have known to look for it.)

Akkana Peck