Lunar South Pole
A combination of Charles Wood's "The Modern Moon: A Personal View", an unexpected mountain top in an image I took, and good friend Bruce McCurdy's comment about a strongly librated full Moon has inspired me to return again and again to the poles of the Moon.
The south pole of the Moon (south up). Image by Tom Bash, featured in LPOD ( Lunar Picture of the Day) https://www2.lpod.org/wiki/December_19,_2007.
Schematic by Ewan Whitekar, featured on p.127 of The Modern Moon and in LPOD https://www2.lpod.org/wiki/May_12,_2007.
Observing this region of the Moon is tricky. You need a strong libration to bring the features into view, as well as a nearly full phase, otherwise the numerous shadows make identification very difficult. Of course at full phase, the Moon is really bright, so observing at high power and using a dark blue filter or even sunglasses is a necessity to avoid eye strain and tearing up. As Bruce McCurdy pointed out to me, the chief characteristic of a strong libration at full phase is the gain in 3D perspective because we are looking "from above, downward at a slant, so we can see the shadows nicely outlining each hilltop". Of course for the south pole we are looking up from underneath, but with an inverted view the brain can make more sense of it. The keys to familiarizing yourself with the terrain is to anchor on Schomberger and Scott (which has the small crater smack on its southside), and to the west anchor on Newton and Casatus (with its sharp white crater inside).
Taken on 20060307_0030UT this image records the surprise chip of light way off the terminator. Not positive, but I think it is M5.
The libration for 20060307 was at extreme SE for that First Quarter image.
The following images during March 2018 were taken with a low resolution (640x480) black and white video camera (that I use for asteroid occultations) and processed in Registax.
Libration starting 20190318. Although a favourable libration in latitude, the Moon this session was rotated a little too much to the east. You can see how this affects the angle between M1, M3, M4, and M5 when comparing Tom Bash's image with mine below.
20190320_05UT FM-1
Cabeus now looks like a bowl. Shadows visible on the north inside part, but the floor is open. Then a drop into darkness before hitting M5. A tiny thin finger of white reaches towards M5 but doesn't quite make it.
M1, the front high point of Cabeus still has a double appearance although less than yesterday. M1's shadow is point right at the gap between M5 and M4. Very low down at the base of M5, there is a thin line reaching out, but not touching the base of M4.
20190321__05UT FM
What a great view across the south polar region. Nice large rolling terrain to the left (east) of M5. Bumps are casting perfect shadows southward. Can still see the top of M1 Cabeus' raised rim at times sports a wrinkle. Their combined shadow is inside the crater, not making it to the back wall though. M5 in the back: the notch in its top is a lot less obvious. M3 is casting its shadow onto M4, beautifully triangular, really fantastic. There is something to the right (west) of M4, a low thin black line. M4 to M5 is continuous light, though the character is different, likely some foreground. And there is a tiny speck above that in the distant background. You can tell from the character of the light that it is behind and not just between the two at the saddle point. In front of M5 there is a black line, a rim just beyond Cabeus but on this side of M5. There is detail in there that comes and goes with the seeing. There is a minor bump on this side of M4 casting its shadow onto M4's base.
20190322_05 UT (FM+1)
The seeing was the worst of this set of nights, so much so that I did not take any notes. The libration has shifted noticeably, as well as the shadows having moved on and pointing slightly eastward. M1 and M3 are now closer to M4 and M5, but that's all I had the patience for.
One month later, the librations are quite similar, but the longitude will be 4 E on the night of full Moon, a little worse.
I will have to wait until spring of 2020 to get another good look at the south pole during full Moon.
I took quite a few south pole videos during the winter of 2007-2008, lots of poor seeing (over heated rooftops and furnace vents), so nothing great. But the view is very similar to that in the spring of 2019 and even more for the upcoming 2020 event.
