Tuesday 24 May 2016
A rare fish from around the South lunar pole.
It’s been four weeks since my last lunar sketch. Not from lack of trying. On those occasions when I was able to pull a scope out, seeing conditions were dreadful. The image through the eyepiece was shimmering and boiling so much it was visible even at 50X magnification.
It was an unexpected long spell of poor seeing, but there was a silver lining to it!
Last Saturday night I set up my telescope in order to fine tune its vibration suppression properties. It was full Moon, and high cloud meant I wasn’t considering doing a sketch. The testing of the new vibration suppression tweak worked, which I’m most pleased about. What I did not expect though was the image I got from the Moon – yet another lesson that needed learning…
Full Moon I had thought of as a shadowless phase of the Moon. But things are not always what we think of at first. The Moon’s orbit around the sun is not in the one flat plane. The Moon orbital plane is a broad one. We can see this over a few nights when we compare its position in the sky when it is at the same point over some landmark. It will be either closer to or further away from that landmark.
The consequence of this, when the Moon is at its highest point in its orbital plane, and it coincides with its full phase, we get a view of the Moon where we can see a thin strip of a terminator. Somewhat like looking under its skirt!
So this Full Moon phase I caught the Moon at, the thinnest strip of the terminator was visible. And what was on view was absolutely staggering! Mountains! Lots and lots of mountains seen not from above, but side on, and with shadows: dark ridges from crater rims, shadows cast back from foreground mountains, & valleys between mountain ranges left in shadow. And beads of light from crater rims just reaching up from the shadows along the terminator. So much going on all along the limb! And the foreshortening, WOW!
The Saturday was not a sketch night, but the following Sunday was crystal clear, and the phase was equivalent from Saturday. And with the thrill of the previous night still fresh in my mind, I knew exactly what I was to chase down – mountains along the limb!
I settled on a spot with a few dramatic mountains were clustered, and a large and highly foreshortened crater lay nearby. The clarity of the night was amazing, and the detail visible on the mountain sides was spectacular. The mountains were not plane white blocks, but textures and variations in illumination were visible. Like looking at snow-capped mountains in the distance. And the foreshortened crater was visible almost like a cross section, where the rise and fall of the ancient rim was appreciable over both the internal and external walls. And as the sketch progressed, those finer details of foreground ridges, mountains and craters emerged, casting their thin shadows backwards.
I felt very privileged to be able to make this piece.
The surprises didn’t stop here for me. Researching the area I sketched, I came to find it was the area around the Moon’s South Pole. The large crater turns out to be Drygalski. Never heard of it? Not surprising really. Most maps won’t show this 150km hole in the Moon. It is visible only for a few days, and only when the Moon’s libration is favourable and positions Drygalski into view.
The Moon’s orbit around the Earth is not a static tidally locked one. The Moon has a wobble. And it is this wobble that actually allows us to see not just 50% of its surface, but 60% of it! Yes, we do get a fleeting glimpse of some of the far side of the Moon.
This Wiki video of one lunar phase, shows the Moon’s libration over the period.
Drygalski is a rare fish, as it is not seen very often, and seeing so much of its floor even more so. Its floor may be visible only two or three times a year for a few nights at a time. I have been able to find very few Earth based photos of Drygalski. A very rare fish.
I hope you enjoy this piece.
Object: Crater Drygalski and Mountains around South Lunar pole.
Scope: C8, 8” SCT
Gear: 8mm LVW, 250X
Date: 22nd May, 2016
Location: Sydney, Australia
Media: Soft pastels & charcoal on A4 size black paper.