by Akkana Peck
Last night, after returning from a hike, I was shocked to notice a rare clear sky with a high crescent moon, so I pulled out the 6" dob for a quick peek before dinner. Which turned into a longer sketching session -- Fracastorius, right on the terminator, was so pretty that I couldn't resist, and as I tried to capture that, the west wall of Piccolomini started peeking out into the sunlight so I had to extend the sketch to cover that as well.
As I sketched, I snuck peeks at other areas of the moon, wondering what that parallel rille complex was, when suddenly I realized -- I was finally seeing the Cauchy Hyperbolas! This structure surrounding the crater Cauchy in the middle of Mare Tranquilitatis consists of Rupes Cauchy and Rima Cauchy, both long, slightly curvedlines which curve away from each other like a pair of shallow hyperbolas. The drawing on Rukl's chart 36 is quite striking, as is the photo of the area in the photo collection, and I'd tried to view the hyperbolas before, without success. I'd seen Rupes Cauchy, in both sunrise and sunset light, but apparently the Rima is very sensitive to sun angle (though it looks very obvious in Rukl's drawing) and had eluded me up to now. Even last night, it was quite a bit less prominent than the Rupes -- but it was evident.
My sketch shows some of the albedo features of this part of Tranquilitatis, like the light streak across the jog in Rima Cauchy, and the dark blotch farther west, neither of which, oddly, are shown in the Rukl chart.
I uploaded the sketches to http://www.shallowsky.com/moonsketch.html
Another interesting view last night was the Humboldt Sea. I noticed a small mare visible in the far northwest, beyond where Atlas and Hercules pointed to the still dark Lacus Mortis, and couldn't remember seeing it before, so I went back to the charts. Chart 7 showed a bit of Mare Humboldtianum, but not the complete circular sea that i was seeing. We must be having a good northeast libration! Sure enough -- 6.6 degrees north, though only 2.57 east. One interesting aspect was the albedo features: the light area in the northwest, near Belkovich, showed just as it does in Rukl's libration chart II, but much more striking was the very light smears in the southwestern part of the mare, which are not shown at all in Rukl's drawing; surprising since they're so obvious and he usually does a good job showing albedo features. Perhaps this coloration only shows up in certain lights. This good libration should last another day, so it's worth checking out this nice view of the Humboldt Sea tonight if your skies are clear (we seem to have clouded up again, alas).